If you're in a field where specific features dominate, then sub-second performance differences likely won't make a difference. Modern web browsers are roughly equivalent in feature support, so performance is one of the deciding factors for users. (If you had a web browser which didn't have CSS support but which was 100x faster in page display and Javascript performance, then I suspect you'll have few users)
Now, orders of magnitude performance differences might affect things, but then that's changing the market.