diff --git a/tests/Standards/AbstractSniffUnitTest.php b/tests/Standards/AbstractSniffUnitTest.php
index 7c6b500e95..ccd90c513c 100644
--- a/tests/Standards/AbstractSniffUnitTest.php
+++ b/tests/Standards/AbstractSniffUnitTest.php
@@ -193,10 +193,13 @@ final public function testSniff()
                 $fixedFile = $testFile.'.fixed';
                 $filename  = basename($testFile);
                 if (file_exists($fixedFile) === true) {
-                    $diff = $phpcsFile->fixer->generateDiff($fixedFile);
-                    if (trim($diff) !== '') {
-                        $fixedFilename     = basename($fixedFile);
-                        $failureMessages[] = "Fixed version of $filename does not match expected version in $fixedFilename; the diff is\n$diff";
+                    if ($phpcsFile->fixer->getContents() !== file_get_contents($fixedFile)) {
+                        // Only generate the (expensive) diff if a difference is expected.
+                        $diff = $phpcsFile->fixer->generateDiff($fixedFile);
+                        if (trim($diff) !== '') {
+                            $fixedFilename     = basename($fixedFile);
+                            $failureMessages[] = "Fixed version of $filename does not match expected version in $fixedFilename; the diff is\n$diff";
+                        }
                     }
                 } else if (is_callable([$this, 'addWarning']) === true) {
                     $this->addWarning("Missing fixed version of $filename to verify the accuracy of fixes, while the sniff is making fixes against the test case file");