It amazes me too. Generally, I've heard that Access starts to bog down after 20 concurrent users and that's on a split database.
I will repeat something I have said before. EVERY database that is either multi-user or supported by someone other than the user NEEDS to be split. This is an absolute rule with me. You are asking for trouble if you don't. While you will get better performance if the front end is on the local PC, its not an absolute necessity.