Well, html5 is newer and hasn't completely made its way through the web yet. And, yes, a lot of people still run older machines but it's a thin line on if you want to create games using html5 or not. I like it, and it's the future because the technology and graphics will only be getting better and better. You should also make sure everything in your game is organised well, not a lot of events running in your current layout by making sure they are destroyed when not on the screen. It's hard to tell why it could be running slow without seeing the game structure. Right now I'm using a piece of crap laptop and my projects run fine on it. I would just go and check how everything is setup. You have to make sure your events are destroyed properly.