At one of the forums I work on I see the session lengh used is 43200 sec (12 hours). I guess that's definitly too long and I will reduce it to like 60-90 minutes. I just wondered if the half day session length might be the reason (or one of them) that the forum is pretty slow? Does that put a lot of strain on CPU / memory?
The proformance hit would probably be in the sessions database table. The rows are purged out when their session expires, so if you have alot of traffic, your sessions table could grow with a long sessions length. Be aware that that your sessions length must be at least as long as your view online time span.