I guess it is because your .mac trial is finished. because all your contact gone through that account.
Now you have the following options: pay and extend for your .mac membership or use an alternative email address with iChat. If you already have an AIM address, you can simly use that one and pay nothing. Or just make a new one.
I am not sure if you have to add all your contacts again, but I think you have to because I think they're not stored in iChat itself.
Why your MBP runs slower should be a complete other question. I really don't think it's related to your iChat problem.
Maybe you've installed something new in the last days which is running in the background, or maybe some automated (time machine) backups?
Just think of everything you changed on your system lately.
Hope that helps!