Enabling HPET in BIOS is just half way of enabling HPET, it needs to be enabled in OS too, and in a way that it's the only timer used.

By default windows uses combination of TSC+ACPI timers, not matter if HPET is enabled in BIOS.

TSC+LAPICs Low performance (slow timers + syncing)
LAPICs low performance (slow timer - no syncing)
TSC+HPET medium performance (slow and fast timer + syncing)
HPET high performance (fast timer - no syncing)

HPET + platformclock=true will give you best timer resolution, frame rate and lowest DPC latency.

You can test timer ratio and QueryPerformanceFrequency with WinTimerTester 1.1 http://www.mediafire...xzo9n84d8lze9nb
The higher the QueryPerformanceFrequency is the better is performance. You only get high frequency with HPET. The other timers will give you significantly less frequency. Also note that if your ratio is not 1.0000 you are off set (or you have wrongly OC'ed), enable HPET and you should be without sync problems.

If you ever want to go back to default timers admin cmd:
bcdedit /deletevalue useplatformclock

Varying depending on setup, one should get increase up to +30 FPS and from the between.Online games is a good example of boost from HPET.