There are no limitations on users unless you're using the free version. Then the limitation is simply with users who may perform edits within the Kentico UI.
From a performance perspective, I'd check your servers and make sure they are scaled properly. I'd suggest your web servers be no less than a quad core, 4GB RAM (8GB would be better) and 60-80GB HDD. For your db servers, again, at least a quad core, with 8GB RAM and 100GB HDD. There should be no issues with having them on 2 separate servers.
Other things to check are the configuration of your sites templates, webparts and custom code. Any of these can cause a lot of slowness even with what is a simplest of configuration changes.
Another thing is to upgrade your sites to v10 or v11. They have some pretty awesome performance enhancements which make the site(s) lightning fast now.
Without seeing the inside of the site, pretty hard to say but I'd start with what I mentioned above.