I usually only use the following speed levels in the Historical Tester:
Test speed = 7
Test speed = 8 (Max)

but the gap between them is quite large. In my own experience, I often need to run visual backtests at intermediate speeds. (i.e. 7.5 etc.)

I know the precise speeds may be platform dependent, but could we reprogram the slider to roughly provide a more useful distribution of test speeds?

Also, the slider is continuous but the underlying speeds are not, which is deceptive. If the slider only supports discrete speeds, then it should probably snap to discrete points.