The choice is ultimately up to you, but of course we recommend that you upgrade from the built-in protection. There's a lot more going on with modern antivirus and you can learn more about how Avast works, with multiple layers, here. You can always give Avast Free a try anytime you like.
2. Will AV slow down my gaming PC?
You shouldn't see any difference at all with a high performance security product. Specific to us, our developers work hard to make sure Avast performs seamlessly, and we're highly rated for it too. The only exception would be during something like a full scan of your hard drive, which will likely have some kind of measurable impact depending on the specs of the machine. The solution there is to schedule active scans during times you are not gaming!
I hope this helps you out, and of course, enjoy your new gaming machine!!