Testing my heavier demo:
Direct Mode + Aero OFF + Mirroring ON = 37.5hz.
Direct Mode + Aero OFF + Mirroring OFF = Image in glasses only updates once every second.
Direct Mode + Aero ON + Mirroring OFF = 37.5hz.
demo used to be 75hz locked before.
I believe there’s something to that one Answerhub question (speculation in comments) that if realworld performance is below 83fps then it drops to 37.5hz. Otherwise it will be at the desired 75hz cap.
Disabling SpeedStep probably delivered just enough juice for my current lighter demo to rise above the threshold needed to lock it to 75hz.