diff --git a/src/workflow/ArcanistDiffWorkflow.php b/src/workflow/ArcanistDiffWorkflow.php
--- a/src/workflow/ArcanistDiffWorkflow.php
+++ b/src/workflow/ArcanistDiffWorkflow.php
@@ -2164,8 +2164,8 @@
     // we always get this 100% right, we're just trying to do something
     // reasonable.
 
-    $local = $this->loadActiveLocalCommitInfo();
-    $hashes = ipull($local, null, 'commit');
+    $hashes = $this->loadActiveDiffLocalCommitHashes();
+    $hashes = array_fuse($hashes);
 
     $usable = array();
     foreach ($commit_messages as $message) {
@@ -2212,8 +2212,8 @@
       return null;
     }
 
-    $local = $this->loadActiveLocalCommitInfo();
-    $hashes = ipull($local, null, 'commit');
+    $hashes = $this->loadActiveDiffLocalCommitHashes();
+    $hashes = array_fuse($hashes);
 
     $usable = array();
     foreach ($messages as $rev => $message) {
@@ -2261,7 +2261,63 @@
     return implode('', $default);
   }
 
-  private function loadActiveLocalCommitInfo() {
+  private function loadActiveDiffLocalCommitHashes() {
+    // The older "differential.querydiffs" method includes the full diff text,
+    // which can be very slow for large diffs. If we can, try to use
+    // "differential.diff.search" instead.
+
+    // We expect this to fail if the Phabricator version on the server is
+    // older than April 2018 (D19386), which introduced the "commits"
+    // attachment for "differential.revision.search".
+
+    // TODO: This can be optimized if we're able to learn the "revisionPHID"
+    // before we get here. See PHI1104.
+
+    try {
+      $revisions_raw = $this->getConduit()->callMethodSynchronous(
+        'differential.revision.search',
+        array(
+          'constraints' => array(
+            'ids' => array(
+              $this->revisionID,
+            ),
+          ),
+        ));
+
+      $revisions = $revisions_raw['data'];
+      $revision = head($revisions);
+      if ($revision) {
+        $revision_phid = $revision['phid'];
+
+        $diffs_raw = $this->getConduit()->callMethodSynchronous(
+          'differential.diff.search',
+          array(
+            'constraints' => array(
+              'revisionPHIDs' => array(
+                $revision_phid,
+              ),
+            ),
+            'attachments' => array(
+              'commits' => true,
+            ),
+            'limit' => 1,
+          ));
+
+        $diffs = $diffs_raw['data'];
+        $diff = head($diffs);
+
+        if ($diff) {
+          $commits = idxv($diff, array('attachments', 'commits', 'commits'));
+          if ($commits !== null) {
+            $hashes = ipull($commits, 'identifier');
+            return array_values($hashes);
+          }
+        }
+      }
+    } catch (Exception $ex) {
+      // If any of this fails, fall back to the older method below.
+    }
+
     $current_diff = $this->getConduit()->callMethodSynchronous(
       'differential.querydiffs',
       array(
@@ -2270,7 +2326,10 @@
     $current_diff = head($current_diff);
 
     $properties = idx($current_diff, 'properties', array());
-    return idx($properties, 'local:commits', array());
+    $local = idx($properties, 'local:commits', array());
+    $hashes = ipull($local, 'commit');
+
+    return array_values($hashes);
   }