When changing prescales to 2, there is a slight increase to 75% of full speed when throttling is enabled, rather than 60%
When I change prescales to 1, then it runs at 100% but this is really foggy picture quality.
Thanks for the assist, I will query this with the mameworld guys, see what they have to say.
I'll keep you in the loop.
I've read that pre-scale is done by the processor (Pre-GPU). Your processor might be underpowered for HLSL.
It also varies by game. The old games that had discrete sound (Frogger for example) are more demanding that other games from the same era.
I have most games set to 4, but had to reduce it on games with discrete sound and 90's 3D games.
It doesn't make much difference to scale it larger than your monitor resolution.
For example, if the original game was 240x320, it doesn't make sense to scale it more than 4x on a 1080 monitor.
MAME scales it to 960, then your video card scales it from 960 to 1080.
At least that's my understanding of how it works and results of my tinkering are consistent with it.