Fixed #10498 (again) -- Made sure the improvements done in r17641 have a smaller impact on speed. Thanks to Anssi Kääriäinen for the patch and Jonas Obrist for reviewing.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17698 bcc190cf-cafb-0310-a4f2-bffc1f526a37