There is literally one single reason old computers are slow, and it's drivers. That is it. Every computer mentioned here is more than capable of running these games fast with a modern browser. The only thing that can make a game unplayably slow is if the driver or graphics card is blacklisted by the browser maker, and then they switch to a really slow software renderer.
This is not a problem with HTML5 itself, it's a problem with drivers, but it is still a problem. I blogged about it here.
Please - before complaining about poor performance first try updating the graphics card driver. That is the most likely thing to fix it. Unfortunately it still does not always fix it, but it gives you the best chance of having a fast running HTML5 game. Unfortunately all your users have to go through that too. But this is not a problem inherent with HTML5: it's solely graphics card maker's fault, for doing a really crap job making unstable drivers. Browser makers don't want your PC crashing as you browse the web so would rather turn off unstable drivers than use them. It sucks, but new computers, or computers with up-to-date drivers, get hardware acceleration then run fine. It's not because new computers are more powerful, it's *just* because new computers have more stable drivers, and old computers tend to have unstable drivers which get disabled by the browser.
t is a hassle and surely much harder than installing an EXE game on a computer
I've argued this to death already, but I genuinely do not understand this at all! So downloading an EXE, going through security warnings and having it only work on one platform is more convenient than just visiting a URL like any other website and the game is already running? Surely not!