Back in 2000, I remember that a PC game I was working on, was slower in NT, than Windows 98 or was it ME. Explanation from Microsoft (they ware the publisher too) was that in NT they had to check every vertex index, whether it's out of bounds, otherwise one somehow might be able to read memory which he's not supposed to. And couple of other things too.

I guess it might be a valid concern. Though I would love if WebGL becomes adopted by everyone.