1) None can beat C2 html5 games with regards to performace. It rocks. Newgrounds, yip you can upload games there no problem at all. Seen a whole lot there.
2) It is completely dependant on the device used to run the game. You would never run Call of duty: Ghosts on a phone (It isn't possible)
But on a browser using a standard pc you won't have issues. Of course, that is if you are sensible about your graphics. A 1000 Huge resolution images of small game assets (bullet, sword, blade of grass) which on screen is 15,15 or similiar is overkill and will land you in trouble.
Sidenote: I love unity, but it has a very very very very very steep learning curve. I used C2 to mockup for unity and fell in love with how simple C2 is. I haven't opened unity for 2 months now - lol