QOS, as the most basic definition, basically prioritizes services to make everything run smoother. So, for instance... If you were streaming netflix, but your roommate was also downloading torrents, you'd want to give Netflix the higher priority so you can watch videos with the best streaming quality possible. And if you had poor cell phone service in your area and you needed Wifi Calling working perfectly, you could set wifi calling to have the highest priority. QOS basically lets you prioritize traffic. This becomes increasingly important with poor internet connections where you don't have enough bandwidth to do everything all at once.
In YOUR case, with a 500/500 connection, you don't have to worry about it. Like, at all. Just turn QOS off and you'll be fine. If you were stressing that 500mb connection at all times, and you needed to prioritize stuff, sure, QOS will work. But I'm willing to bet a lot of money you won't need it at all.
To answer your CPU question: if you're just using your router as a router, you don't have to worry about it. And, without trying to sound like a jerk, if you're asking the basic questions here, I DON'T think you're going to be doing anything advanced with your router, I wouldn't worry about it at all. The CPU comes into play when you're doing intensive work on the router, like running scripts, using the VPN client, plus using the router's USB to connect to a USB hard drive mapped as a network drive, etc.