I can't see PHP sessions slowing down your site by that amount. As
someone said, it should be no more than a split second. If you are
having that much of a problem with them, then I would say it is either
your implementation, or another determining factor.

I would not, personally, stray away from PHP's GC of sessions. I would
imagine that any third-party concoction could not be any faster. I
would however, check to see how long PHP takes to clean up defunct
sessions, and also monitor how many sessions are lying around at any
given time.

Regards,
- Craige