diff --git a/CHANGELOG b/CHANGELOG
index 145ed4474..57528c29a 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -63,6 +63,7 @@ CHANGELOG Roundcube Webmail
 - Fix untagged COPYUID responses handling - again (#5982)
 - Fix PHP warning "idn_to_utf8(): INTL_IDNA_VARIANT_2003 is deprecated" with PHP 7.2 (#6075)
 - Fix bug where Archive folder wasn't auto-created on login with create_default_folders=true
+- Fix performance issue when parsing malformed and long Date header (#6087)
 
 RELEASE 1.3.3
 -------------
diff --git a/program/lib/Roundcube/rcube_utils.php b/program/lib/Roundcube/rcube_utils.php
index 4cae55999..70a459845 100644
--- a/program/lib/Roundcube/rcube_utils.php
+++ b/program/lib/Roundcube/rcube_utils.php
@@ -729,15 +729,19 @@ class rcube_utils
             return (int) $date;
         }
 
+        // It can be very slow when provided string is not a date and very long
+        if (strlen($date) > 128) {
+            $date = substr($date, 0, 128);
+        }
+
         // if date parsing fails, we have a date in non-rfc format.
         // remove token from the end and try again
-        while ((($ts = @strtotime($date . $tzname)) === false) || ($ts < 0)) {
-            $d = explode(' ', $date);
-            array_pop($d);
-            if (!$d) {
+        while (($ts = @strtotime($date . $tzname)) === false || $ts < 0) {
+            if (($pos = strrpos($date, ' ')) === false) {
                 break;
             }
-            $date = implode(' ', $d);
+
+            $date = rtrim(substr($date, 0, $pos));
         }
 
         return (int) $ts;