diff --git a/src/applications/diffusion/conduit/DiffusionInternalGitRawDiffQueryConduitAPIMethod.php b/src/applications/diffusion/conduit/DiffusionInternalGitRawDiffQueryConduitAPIMethod.php
index 1e4c906322..ac241a282f 100644
--- a/src/applications/diffusion/conduit/DiffusionInternalGitRawDiffQueryConduitAPIMethod.php
+++ b/src/applications/diffusion/conduit/DiffusionInternalGitRawDiffQueryConduitAPIMethod.php
@@ -1,74 +1,123 @@
 <?php
 
 final class DiffusionInternalGitRawDiffQueryConduitAPIMethod
   extends DiffusionQueryConduitAPIMethod {
 
   public function isInternalAPI() {
     return true;
   }
 
   public function getAPIMethodName() {
     return 'diffusion.internal.gitrawdiffquery';
   }
 
   public function getMethodDescription() {
     return pht('Internal method for getting raw diff information.');
   }
 
   protected function defineReturnType() {
     return 'string';
   }
 
   protected function defineCustomParamTypes() {
     return array(
       'commit' => 'required string',
     );
   }
 
   protected function getResult(ConduitAPIRequest $request) {
     $drequest = $this->getDiffusionRequest();
     $repository = $drequest->getRepository();
+    $commit = $request->getValue('commit');
 
     if (!$repository->isGit()) {
       throw new Exception(
         pht(
           'This API method can only be called on Git repositories.'));
     }
 
     // Check if the commit has parents. We're testing to see whether it is the
     // first commit in history (in which case we must use "git log") or some
     // other commit (in which case we can use "git diff"). We'd rather use
     // "git diff" because it has the right behavior for merge commits, but
     // it requires the commit to have a parent that we can diff against. The
     // first commit doesn't, so "commit^" is not a valid ref.
     list($parents) = $repository->execxLocalCommand(
       'log -n1 --format=%s %s',
       '%P',
-      $request->getValue('commit'));
-
+      $commit);
     $use_log = !strlen(trim($parents));
+
+    // First, get a fast raw diff without "--find-copies-harder". This flag
+    // produces better results for moves and copies, but is explosively slow
+    // for large changes to large repositories. See T10423.
+    $raw = $this->getRawDiff($repository, $commit, $use_log, false);
+
+    // If we got a normal-sized diff (no more than 100 modified files), we'll
+    // try using "--find-copies-harder" to improve the output. This improved
+    // output is mostly useful for small changes anyway.
+    $try_harder = (substr_count($raw, "\n") <= 100);
+    if ($try_harder) {
+      try {
+        $raw = $this->getRawDiff($repository, $commit, $use_log, true);
+      } catch (Exception $ex) {
+        // Just ignore any exception we hit, we'll use the fast output
+        // instead.
+      }
+    }
+
+    return $raw;
+  }
+
+  private function getRawDiff(
+    PhabricatorRepository $repository,
+    $commit,
+    $use_log,
+    $try_harder) {
+
+    $flags = array(
+      '-n1',
+      '-M',
+      '-C',
+      '-B',
+      '--raw',
+      '-t',
+      '--abbrev=40',
+    );
+
+    if ($try_harder) {
+      $flags[] = '--find-copies-harder';
+    }
+
     if ($use_log) {
       // This is the first commit so we need to use "log". We know it's not a
       // merge commit because it couldn't be merging anything, so this is safe.
 
       // NOTE: "--pretty=format: " is to disable diff output, we only want the
       // part we get from "--raw".
-      list($raw) = $repository->execxLocalCommand(
-        'log -n1 -M -C -B --find-copies-harder --raw -t '.
-          '--pretty=format: --abbrev=40 %s',
-        $request->getValue('commit'));
+      $future = $repository->getLocalCommandFuture(
+        'log %Ls --pretty=format: %s',
+        $flags,
+        $commit);
     } else {
       // Otherwise, we can use "diff", which will give us output for merges.
       // We diff against the first parent, as this is generally the expectation
       // and results in sensible behavior.
-      list($raw) = $repository->execxLocalCommand(
-        'diff -n1 -M -C -B --find-copies-harder --raw -t '.
-          '--abbrev=40 %s^1 %s',
-        $request->getValue('commit'),
-        $request->getValue('commit'));
+      $future = $repository->getLocalCommandFuture(
+        'diff %Ls %s^1 %s',
+        $flags,
+        $commit,
+        $commit);
+    }
+
+    // Don't spend more than 30 seconds generating the slower output.
+    if ($try_harder) {
+      $future->setTimeout(30);
     }
 
+    list($raw) = $future->resolvex();
+
     return $raw;
   }
 
 }