anyone written a device performance probe?

Different mobile phones run different FPS. Has anyone done anything in JS to detect the relative speed of the device? I have seem some JS which just loops for a few MS to see if its an iphone 3 (very slow)or iphone 4 for example.

Obviously I can use the System-> Is on mobile device, but this doesnt tell me if if Im running say ipad3 (which is very fast) or an ipad 1 (which I need to no do some animations etc.)

Any ideas?

I could just do a loop a 100 times and see how long it took, but you never know what the phone is currently doing.

I use the browser string to see if its an iphone or ipad at least, but you cant do much more than that.

This is the standard performance test we use to get an impression of a device's performance. It takes about a minute to run. You should not make a performance test that measures event performance, because most games spend 80-90% of their time in rendering.

If you're trying to measure performance on startup, there's no good and reliable way to do that - just have a setting for effects, e.g. low, medium or high.