I guess it depends on what you mean by freezing and really slow. Diagnosing mobile performance issues is a nightmare (they can be highly variable by nature), but first we need to establish whether it's a mobile problem or a device one.
I always start by ruling out WiFi and conducting tests via a wired link to one of those ports. After that it would be good to see if you can still access the router's admin control panel when the "freezing" occurs, as if so then it may be on the mobile side.
If it's on three there's been a lot of discussion of issues like connections hanging - ie go to a web page and occasionally it just doesn't load. I kept getting the same and terminated my 3 account. Notable on vodafone no such issue.
But there are many possible issues with mobile, or indeed any internet, so can you describe the symptoms in more detail?