I saw many people use WordPress as a CMS, is that common? but the problem I noticed is that the download in such sites is slow, is WordPress slow?
Wordpress IS a CMS. It hasn't been just a blogging platform for a long time.
And no, it's not slow if you don't make it slow. WP out of the box is really fast, but people slow it down with poorly-written plugins and themes.
You can do whatever you want with Wordpress, as it has hooks for basically everything. And you can find a lot of information in the Codex. There are also tons of tutorials online.