What sort of server are you on? Do you run it yourself or are on on a hosting account with another company?
My clients run a few threads sites on my servers, and they all go great guns. However the server is important. On my old server, it was not so brilliant - I will be the first to admit it - but it was down to the hardware not threads.
We can't directly help you with your issues with IP - but you will find plenty of first class mods in the modifications forum above.
As far as the logging in issues, these are usually connected with either the referrer issue (they are running a firewall), or they have corrupt cookies.
at the moment - but expanding all the time....) 
