What's the Catch

Restricting processes to certain cores just shifts the usage to those cores unless a core is maxed out. At max CPU usage programs take longer to do all sorts of things and this shows as visible lag. For every second your file loads faster, something on your system had to wait to be able to access it's own files. Sometimes it's visible, especially if the resources were adjusted improperly, most of the time it isn't. Regardless, if you're gaming, it's unlikely you'll notice other programs lag as your focused on a single program, the game.