There is probably business strategy in that, but I remember flash on my mac to be terrible (100% cpu, draining battery like crazy). While the same version on windows was running perfectly.
Try WebGL on any Mac (Safari is the worst performer of them all btw). You will see a bit of lag, frame drops, CPUs at 100%, a lot heat and your battery will die in two hours max.
Open the same site on a Chrome/Windows machine (with comparable specs of course) and you don't see any of that trouble.
Why? Because Apple wants everything done "The Apple Way®©™" and other people just don't care so they just create a substandard implementation and call it a day. And it's only going to get worse now that they want to push Metal everywhere.