diff --git a/src/rebar_compiler_dag.erl b/src/rebar_compiler_dag.erl
index db903b16..74e97984 100644
--- a/src/rebar_compiler_dag.erl
+++ b/src/rebar_compiler_dag.erl
@@ -7,17 +7,19 @@
 
 -include("rebar.hrl").
 
--define(DAG_VSN, 3).
+-define(DAG_VSN, 4).
 -define(DAG_ROOT, "source").
 -define(DAG_EXT, ".dag").
 
--type dag_v() :: {digraph:vertex(), term()} | 'false'.
--type dag_e() :: {digraph:vertex(), digraph:vertex()}.
--type critical_meta() :: term(). % if this changes, the DAG is invalid
--type dag_rec() :: {list(dag_v()), list(dag_e()), critical_meta()}.
--type dag() :: digraph:graph().
+-type critical_meta() :: term().
+
 -record(dag, {vsn = ?DAG_VSN :: pos_integer(),
-              info = {[], [], []} :: dag_rec()}).
+    meta :: critical_meta(),
+    vtab :: notable | [tuple()],
+    etab :: notable | [tuple()],
+    ntab :: notable | [tuple()]}).
+
+-type dag() :: digraph:graph().
 
 %% @doc You should initialize one DAG per compiler module.
 %% `CritMeta' is any contextual information that, if it is found to change,
@@ -105,13 +107,46 @@ filter_prefix(G, [{App, Out} | AppTail] = AppPaths, [File | FTail]) ->
             filter_prefix(G, AppPaths, FTail)
     end.
 
+finalise_populate_sources(_G, _InDirs, Waiting) when Waiting =:= #{} ->
+    ok;
+finalise_populate_sources(G, InDirs, Waiting) ->
+    %% wait for all deps to complete
+    receive
+        {deps, Pid, AbsIncls} ->
+            {Status, Source} = maps:get(Pid, Waiting),
+            %% the file hasn't been visited yet; set it to existing, but with
+            %% a last modified value that's null so it gets updated to something new.
+            [digraph:add_vertex(G, Src, 0) || Src <- AbsIncls,
+                digraph:vertex(G, Src) =:= false],
+            %% drop edges from deps that aren't included!
+            [digraph:del_edge(G, Edge) || Status == old,
+                Edge <- digraph:out_edges(G, Source),
+                {_, _Src, Path, _Label} <- [digraph:edge(G, Edge)],
+                not lists:member(Path, AbsIncls)],
+            %% Add the rest
+            [digraph:add_edge(G, Source, Incl) || Incl <- AbsIncls],
+            %% mark the digraph dirty when there is any change in
+            %%  dependencies, for any application in the project
+            mark_dirty(G),
+            finalise_populate_sources(G, InDirs, Waiting);
+        {'DOWN', _MRef, process, Pid, normal} ->
+            finalise_populate_sources(G, InDirs, maps:remove(Pid, Waiting));
+        {'DOWN', _MRef, process, Pid, Reason} ->
+            {_Status, Source} = maps:get(Pid, Waiting),
+            ?ERROR("Failed to get dependencies for ~s~n~p", [Source, Reason]),
+            ?FAIL
+    end.
+
 %% @doc this function scans all the source files found and looks into
 %% all the `InDirs' for deps (other source files, or files that aren't source
 %% but still returned by the compiler module) that are related
 %% to them.
-populate_sources(_G, _Compiler, _InDirs, [], _DepOpts) ->
-    ok;
-populate_sources(G, Compiler, InDirs, [Source|Erls], DepOpts) ->
+populate_sources(G, Compiler, InDirs, Sources, DepOpts) ->
+    populate_sources(G, Compiler, InDirs, Sources, DepOpts, #{}).
+
+populate_sources(G, _Compiler, InDirs, [], _DepOpts, Waiting) ->
+    finalise_populate_sources(G, InDirs, Waiting);
+populate_sources(G, Compiler, InDirs, [Source|Erls], DepOpts, Waiting) ->
     case digraph:vertex(G, Source) of
         {_, LastUpdated} ->
             case filelib:last_modified(Source) of
@@ -120,21 +155,20 @@ populate_sources(G, Compiler, InDirs, [Source|Erls], DepOpts) ->
                     %% from the graph.
                     digraph:del_vertex(G, Source),
                     mark_dirty(G),
-                    populate_sources(G, Compiler, InDirs, Erls, DepOpts);
+                    populate_sources(G, Compiler, InDirs, Erls, DepOpts, Waiting);
                 LastModified when LastUpdated < LastModified ->
                     digraph:add_vertex(G, Source, LastModified),
-                    prepopulate_deps(G, Compiler, InDirs, Source, DepOpts, old),
-                    mark_dirty(G);
+                    Worker = prepopulate_deps(Compiler, InDirs, Source, DepOpts, self()),
+                    populate_sources(G, Compiler, InDirs, Erls, DepOpts, Waiting#{Worker => {old, Source}});
                 _ -> % unchanged
-                    ok
+                    populate_sources(G, Compiler, InDirs, Erls, DepOpts, Waiting)
             end;
         false ->
             LastModified = filelib:last_modified(Source),
             digraph:add_vertex(G, Source, LastModified),
-            prepopulate_deps(G, Compiler, InDirs, Source, DepOpts, new),
-            mark_dirty(G)
-    end,
-    populate_sources(G, Compiler, InDirs, Erls, DepOpts).
+            Worker = prepopulate_deps(Compiler, InDirs, Source, DepOpts, self()),
+            populate_sources(G, Compiler, InDirs, Erls, DepOpts, Waiting#{Worker => {new, Source}})
+    end.
 
 %% @doc Scan all files in the digraph that are seen as dependencies, but are
 %% neither source files nor artifacts (i.e. header files that don't produce
@@ -228,9 +262,13 @@ restore_dag(G, File, CritMeta) ->
         {ok, Data} ->
             %% The CritMeta value is checked and if it doesn't match, we fail
             %% the whole restore operation.
-            #dag{vsn=?DAG_VSN, info={Vs, Es, CritMeta}} = binary_to_term(Data),
-            [digraph:add_vertex(G, V, LastUpdated) || {V, LastUpdated} <- Vs],
-            [digraph:add_edge(G, V1, V2, Label) || {_, V1, V2, Label} <- Es],
+            #dag{vsn=?DAG_VSN, meta = CritMeta, vtab = VTab,
+                etab = ETab, ntab = NTab} = binary_to_term(Data),
+            {digraph, VT, ET, NT, false} = G,
+            true = ets:insert_new(VT, VTab),
+            true = ets:insert_new(ET, ETab),
+            true = ets:delete_all_objects(NT),
+            true = ets:insert(NT, NTab),
             ok;
         {error, _Err} ->
             ok
@@ -238,9 +276,9 @@ restore_dag(G, File, CritMeta) ->
 
 store_dag(G, File, CritMeta) ->
     ok = filelib:ensure_dir(File),
-    Vs = lists:map(fun(V) -> digraph:vertex(G, V) end, digraph:vertices(G)),
-    Es = lists:map(fun(E) -> digraph:edge(G, E) end, digraph:edges(G)),
-    Data = term_to_binary(#dag{info={Vs, Es, CritMeta}}, [{compressed, 2}]),
+    {digraph, VT, ET, NT, false} = G,
+    Data = term_to_binary(#dag{meta = CritMeta, vtab = ets:tab2list(VT),
+        etab = ets:tab2list(ET), ntab = ets:select(NT, [{'_',[],['$_']}])}, [{compressed, 2}]),
     file:write_file(File, Data).
 
 %% Drop a file from the digraph if it doesn't exist, and if so,
@@ -285,26 +323,20 @@ maybe_rm_vertex(G, Source) ->
 %% mark its timestamp to 0, which means we have no info on it.
 %% Source files will be covered at a later point in their own scan, and
 %% non-source files are going to be covered by `populate_deps/3'.
-prepopulate_deps(G, Compiler, InDirs, Source, DepOpts, Status) ->
-    SourceDir = filename:dirname(Source),
-    AbsIncls = case erlang:function_exported(Compiler, dependencies, 4) of
-        false ->
-            Compiler:dependencies(Source, SourceDir, InDirs);
-        true ->
-            Compiler:dependencies(Source, SourceDir, InDirs, DepOpts)
-    end,
-    %% the file hasn't been visited yet; set it to existing, but with
-    %% a last modified value that's null so it gets updated to something new.
-    [digraph:add_vertex(G, Src, 0) || Src <- AbsIncls,
-                                      digraph:vertex(G, Src) =:= false],
-    %% drop edges from deps that aren't included!
-    [digraph:del_edge(G, Edge) || Status == old,
-                                  Edge <- digraph:out_edges(G, Source),
-                                  {_, _Src, Path, _Label} <- [digraph:edge(G, Edge)],
-                                  not lists:member(Path, AbsIncls)],
-    %% Add the rest
-    [digraph:add_edge(G, Source, Incl) || Incl <- AbsIncls],
-    ok.
+prepopulate_deps(Compiler, InDirs, Source, DepOpts, Control) ->
+    {Worker, _MRef} = spawn_monitor(
+        fun () ->
+            SourceDir = filename:dirname(Source),
+            AbsIncls = case erlang:function_exported(Compiler, dependencies, 4) of
+                false ->
+                    Compiler:dependencies(Source, SourceDir, InDirs);
+                true ->
+                    Compiler:dependencies(Source, SourceDir, InDirs, DepOpts)
+            end,
+            Control ! {deps, self(), AbsIncls}
+        end
+    ),
+    Worker.
 
 %% check that a dep file is up to date
 refresh_dep(_G, {artifact, _}) ->
diff --git a/src/rebar_compiler_epp.erl b/src/rebar_compiler_epp.erl
index 121e6f31..5334ea15 100644
--- a/src/rebar_compiler_epp.erl
+++ b/src/rebar_compiler_epp.erl
@@ -4,8 +4,18 @@
 %%% @end
 -module(rebar_compiler_epp).
 -export([deps/2, resolve_module/2]).
+%% cache (a la code path storage, but for dependencies not in code path)
+-export([ensure_started/0, flush/0, resolve_source/2]).
+-export([init/1, handle_call/3, handle_cast/2]).
+%% remove when OTP 19 support is no longer needed
+-export([handle_info/2, terminate/2, code_change/3]).
+
+-behaviour(gen_server).
+
 -include_lib("kernel/include/file.hrl").
 
+-include("rebar.hrl").
+
 %%%%%%%%%%%%%%%%%%%%%%%%%%%
 %%% Basic File Handling %%%
 %%%%%%%%%%%%%%%%%%%%%%%%%%%
@@ -37,6 +47,120 @@ resolve_module(Mod, Paths) ->
         Path -> {ok, Path}
     end.
 
+%%%%%%%%%%%%%%%%%%%%%%%%%%%
+%%% Cache for deps      %%%
+%%%%%%%%%%%%%%%%%%%%%%%%%%%
+-spec ensure_started() -> ok.
+ensure_started() ->
+    case whereis(?MODULE) of
+        undefined ->
+            case gen_server:start({local, ?MODULE}, ?MODULE, [], []) of
+                {ok, _Pid} ->
+                    ok;
+                {error, {already_started, _Pid}} ->
+                    ok
+            end;
+        Pid when is_pid(Pid) ->
+            ok
+    end.
+
+flush() ->
+    gen_server:cast(?MODULE, flush).
+
+%% @doc Resolves "Name" erl module to a path, given list of paths to search.
+%% Caches result for subsequent requests.
+-spec resolve_source(atom() | file:filename_all(), [file:filename_all()]) -> {true, file:filename_all()} | false.
+resolve_source(Name, Dirs) when is_atom(Name) ->
+    gen_server:call(?MODULE, {resolve, atom_to_list(Name) ++ ".erl", Dirs});
+resolve_source(Name, Dirs) when is_list(Name) ->
+    gen_server:call(?MODULE, {resolve, Name, Dirs}).
+
+-record(state, {
+    %% filesystem cache, denormalised
+    fs = #{} :: #{file:filename_all() => [file:filename_all()]},
+    %% map of module name => abs path
+    resolved = #{} :: #{file:filename_all() => file:filename_all()}
+}).
+
+init([]) ->
+    {ok, #state{}}.
+
+handle_call({resolve, Name, Dirs}, _From, #state{fs = Fs, resolved = Res} = State) ->
+    case maps:find(Name, Res) of
+        {ok, Found} ->
+            {reply, Found, State};
+        error ->
+            {Resolved, NewFs} = resolve(Name, Fs, Dirs),
+            {reply, Resolved, State#state{resolved = Res#{Name => Resolved}, fs = NewFs}}
+    end.
+
+handle_cast(flush, _State) ->
+    {noreply, #state{}}.
+
+resolve(_Name, Fs, []) ->
+    {false, Fs};
+resolve(Name, Fs, [Dir | Tail]) ->
+    {NewFs, Files} = list_directory(Dir, Fs),
+    case lists:member(Name, Files) of
+        true ->
+            {{true, filename:join(Dir, Name)}, NewFs};
+        false ->
+            resolve(Name, NewFs, Tail)
+    end.
+
+%% list_directory/2 caches files in the directory and all subdirectories,
+%%  to support the behaviour of looking for source files in
+%%  subdirectories of src/* folder.
+%% This may introduce weird dependencies for cases when CT
+%%  test cases contain test data with files named the same
+%%  as requested behaviour/parse_transforms, but let's hope
+%%  it won't happen for many projects. If it does, in fact,
+%%  it won't cause any damage, just extra unexpected recompiles.
+list_directory(Dir, Cache) ->
+    case maps:find(Dir, Cache) of
+        {ok, Files} ->
+            {Cache, Files};
+        error ->
+            case file:list_dir(Dir) of
+                {ok, DirFiles} ->
+                    %% create a full list of *.erl files under Dir.
+                    {NewFs, Files} = lists:foldl(
+                        fun (File, {DirCache, Files} = Acc) ->
+                            %% recurse into subdirs
+                            FullName = filename:join(Dir, File),
+                            case filelib:is_dir(FullName) of
+                                true ->
+                                    {UpdFs, MoreFiles} = list_directory(FullName, DirCache),
+                                    {UpdFs, MoreFiles ++ Files};
+                                false ->
+                                    %% ignore all but *.erl files
+                                    case filename:extension(File) =:= ".erl" of
+                                        true ->
+                                            {DirCache, [File | Files]};
+                                        false ->
+                                            Acc
+                                    end
+                            end
+                        end,
+                        {Cache, []}, DirFiles),
+                    {NewFs#{Dir => Files}, Files};
+                {error, Reason} ->
+                    ?DEBUG("Failed to list ~s, ~p", [Dir, Reason]),
+                    {Cache, []}
+            end
+    end.
+
+%%%%%%%%%%%%%%%
+%%% OTP 19  %%%
+handle_info(_Request, State) ->
+    {noreply, State}.
+
+terminate(_Reason, _State) ->
+    ok.
+
+code_change(_OldVsn, State, _Extra) ->
+    {ok, State}.
+
 %%%%%%%%%%%%%%%
 %%% PRIVATE %%%
 %%%%%%%%%%%%%%%
diff --git a/src/rebar_compiler_erl.erl b/src/rebar_compiler_erl.erl
index 9a8a9377..88783ab0 100644
--- a/src/rebar_compiler_erl.erl
+++ b/src/rebar_compiler_erl.erl
@@ -100,6 +100,7 @@ dependencies(Source, SourceDir, Dirs) ->
     end.
 
 dependencies(Source, _SourceDir, Dirs, DepOpts) ->
+    rebar_compiler_epp:ensure_started(),
     OptPTrans = proplists:get_value(parse_transforms, DepOpts, []),
     try rebar_compiler_epp:deps(Source, DepOpts) of
         #{include := AbsIncls,
@@ -110,9 +111,9 @@ dependencies(Source, _SourceDir, Dirs, DepOpts) ->
             %% TODO: check for core transforms?
             {_MissIncl, _MissInclLib} =/= {[],[]} andalso
             ?DEBUG("Missing: ~p", [{_MissIncl, _MissInclLib}]),
-            expand_file_names([module_to_erl(Mod) || Mod <- OptPTrans ++ PTrans], Dirs) ++
-            expand_file_names([module_to_erl(Mod) || Mod <- Behaviours], Dirs) ++
-            AbsIncls
+            lists:filtermap(
+                fun (Mod) -> rebar_compiler_epp:resolve_source(Mod, Dirs) end,
+                OptPTrans ++ PTrans ++ Behaviours) ++ AbsIncls
     catch
         error:{badmatch, {error, Reason}} ->
             case file:format_error(Reason) of
@@ -141,6 +142,7 @@ compile(Source, [{_, OutDir}], Config, ErlOpts) ->
     end.
 
 compile_and_track(Source, [{Ext, OutDir}], Config, ErlOpts) ->
+    rebar_compiler_epp:flush(),
     BuildOpts = [{outdir, OutDir} | ErlOpts],
     Target = target_base(OutDir, Source) ++ Ext,
     AllOpts = case erlang:function_exported(compile, env_compiler_options, 0) of