[1.2.X] Fixed #14697 - speeded up model instance creation by moving work outside of loops

Thanks to akaariai for the report and initial patch.

Backport of [14687] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14688 bcc190cf-cafb-0310-a4f2-bffc1f526a37