Based on the % game speed in the stats window, it seems to start at +6 with about a 20-30% speed boost.
It actually may be happening at lower speeds but it's not enough that I notice.
For the record at +10, my game goes from about 250% game speed with mouse not moving to 675% game speed with mouse moving. Based on the speed of the seconds counting up on the game clock, I think these game speeds are correct for my game.
My $0.02 on this issue.
D.