diff --git a/src/applications/differential/engine/DifferentialChangesetEngine.php b/src/applications/differential/engine/DifferentialChangesetEngine.php
--- a/src/applications/differential/engine/DifferentialChangesetEngine.php
+++ b/src/applications/differential/engine/DifferentialChangesetEngine.php
@@ -88,6 +88,20 @@
 
 
   private function detectCopiedCode(array $changesets) {
+    // See PHI944. If the total number of changed lines is excessively large,
+    // don't bother with copied code detection. This can take a lot of time and
+    // memory and it's not generally of any use for very large changes.
+    $max_size = 65535;
+
+    $total_size = 0;
+    foreach ($changesets as $changeset) {
+      $total_size += ($changeset->getAddLines() + $changeset->getDelLines());
+    }
+
+    if ($total_size > $max_size) {
+      return;
+    }
+
     $min_width = 30;
     $min_lines = 3;