There are several things you can do to speed up your DNN in general - besides a number of threads here, check out Mitchel Sellers' blog, and, also do some Google searching. The topic is so widely discussed you will have no problem finding all sorts of discussions.

As far as your installation slowing down... It's a problem with IIS and your server. Check running processes to determine which one is causing your processor to slow down.

Installing modules in itself is not going to slow DNN down - it does so on the first run after installation due to specifics of ASP.NET, but after first page it shouldn't be affected anymore. Of course if the module is poorely written it might create all sorts of problems... but, most likely, its processes running on your server that are causing the issue.

You probably mean a LOOOOOOOOOOOOOT of modules - some rather unusual amount? Also, how would it create a problem if it's just installed? (not actually sitting on one page all doing something...)