I wouldn't worry about performance so much. You can use the free version of the engine and check out some example projects, including stress tests.
As far as what kind of games can be made with construct, the answer is pretty much any 2d game you can think up.
To master the engine is much like mastering any other tool, and that is simply using it, reading the manual, and learning from others in the community.
Learning JS would also be useful.