That would be a way to deal with the problem from the opposite end, but if this solution works I’ll leave it alone.
Viewing post in Added an HTML5 demo (and made the demo fancier) comments
By your own definition it doesn't work accurately other on really slow machines. The previous test will always affect the result for the current test. Test1 will always have a higher FPS so the first readings for Test2 will be artificially higher than real - repeat this observation as it will hold true for each test
Like I said, i built in a delay between rounds that should wait out the time it takes for the sliding window to catch up.
That said, I checked the HTML5 source and it apparently takes two seconds for the fps value to refresh and I only accounted for one, so I edited it again.