Run DUB in a separate process as well, so it will not cause import lag.

ale_linters/d/dmd.vim

@@ -7,7 +7,8 @@ let g:loaded_ale_linters_d_dmd = 1
 " A function for finding the dmd-wrapper script in the Vim runtime paths
 function! s:FindWrapperScript()
     for parent in split(&runtimepath, ',')
-        let path = parent . '/' . 'dmd-wrapper'
+        " Expand the path to deal with ~ issues.
+        let path = expand(parent . '/' . 'dmd-wrapper')
 
         if filereadable(path)
             return path
@@ -15,48 +16,11 @@ function! s:FindWrapperScript()
     endfor
 endfunction
 
-function! ale_linters#d#dmd#GetDubImports(buffer)
-    if !executable('dub')
-        " If we don't have dub, then stop here.
-        return []
-    endif
-
-    " Try to find dub.json
-    let dub_path = findfile("dub.json", ",;")
-
-    if dub_path == ''
-        " Try to find package.json if that fails
-        let dub_path = findfile("package.json", ",;")
-    endif
-
-    if dub_path == ''
-        " We couldn't find the project root directory, so give up.
-        return
-    endif
-
-    let dub_dir = fnamemodify(dub_path, ':h')
-    let old_path = getcwd()
-
-    try
-        " Temporarily change to the project directory.
-        execute 'cd' . fnameescape(dub_dir)
-
-        return split(system('dub describe --import-paths'), '\n')
-    finally
-        " Change back to the old path.
-        execute 'cd' . fnameescape(old_path)
-    endtry
-endfunction
-
 function! ale_linters#d#dmd#GetCommand(buffer)
     let wrapper_script = s:FindWrapperScript()
 
     let command = wrapper_script . ' -o- -vcolumns -c'
 
-    for path in ale_linters#d#dmd#GetDubImports(a:buffer)
-        let command .= ' -I' . shellescape(path)
-    endfor
-
     return command
 endfunction
 

dmd-wrapper

@@ -13,4 +13,27 @@ while read line; do
     echo "$line" >> "$temp_file"
 done
 
-dmd "$@" "$temp_file"
+# Read imports from DUB.
+original_path="$(readlink -m .)"
+path="$original_path"
+import_line_options=''
+
+# We need to look for variable configuration files in parent directories.
+while [ "$path" != '/' ]; do
+    if [ -f "$path/dub.sdl" ] || [ -f "$path/dub.json" ] || [ -f "$path/package.json" ]; then
+
+        cd "$path"
+
+         while read import_line; do
+            import_line_options="$import_line_options -I$import_line"
+        done <<< "$(dub describe --import-paths)"
+
+        cd "$original_path"
+
+        break
+    fi
+
+    path="$(dirname "$path")"
+done
+
+dmd $import_line_options "$@" "$temp_file"