diff --git a/lib/minify/matthiasmullie-minify/src/CSS.php b/lib/minify/matthiasmullie-minify/src/CSS.php
index a9a70a0460a..a66dfe0353e 100644
--- a/lib/minify/matthiasmullie-minify/src/CSS.php
+++ b/lib/minify/matthiasmullie-minify/src/CSS.php
@@ -636,7 +636,35 @@ class CSS extends Minify
 
             return $placeholder;
         };
+        // Moodle-specific change MDL-68191 starts.
+        /* This was the old code:
         $this->registerPattern('/\n?\/\*(!|.*?@license|.*?@preserve).*?\*\/\n?/s', $callback);
+        */
+        // This is the new, more accurate and faster regex.
+        $this->registerPattern('/
+            # optional newline
+            \n?
+
+            # start comment
+            \/\*
+
+            # comment content
+            (?:
+                # either starts with an !
+                !
+            |
+                # or, after some number of characters which do not end the comment
+                (?:(?!\*\/).)*?
+
+                # there is either a @license or @preserve tag
+                @(?:license|preserve)
+            )
+
+            # then match to the end of the comment
+            .*?\*\/\n?
+
+            /ixs', $callback);
+        // Moodle-specific change MDL-68191.
 
         $this->registerPattern('/\/\*.*?\*\//s', '');
     }
diff --git a/lib/minify/readme_moodle.txt b/lib/minify/readme_moodle.txt
index 9593ad066d5..d03fa3d894c 100644
--- a/lib/minify/readme_moodle.txt
+++ b/lib/minify/readme_moodle.txt
@@ -14,3 +14,10 @@ Local changes applied:
 MDL-67115: php 74 compliance - implode() params order. Note this has been fixed upstream
   by https://github.com/matthiasmullie/minify/pull/300 so, whenever this library is updated
   check if the fix is included and remove this note.
+
+MDL-68191: https://github.com/matthiasmullie/minify/issues/317 is a bug that stops
+  large sections of the CSS from being minimised, and also is a huge performance drain.
+  We have applied the fix sent upstream because the performance win is so big.
+  (E.g. one case I measured, with the bug was 40 seconds to minify CSS, with the fix was
+  a few seconds. This is one of the reasons Behat runs in the browser are so slow.)
+  Whenever this library is updated check if the fix is included and remove this note.