My theory is the fps increase because it's reducing the rate at which all fixed calculations happen first. Usually physics in Unity, probably more stuff like (morphs?) in VaM. If they are tied to a short timer (1 sec / 72 Hz fixed rate = 13,88 milliseconds max per fixed frame) and MUST be finished to present a new frame then these would be the first affected by process time manipulation. With the time "stretched" the process now has suddenly 27,7 milliseconds for a fixed frame.
With less time spend for fixed calculations on the CPU > lower frame time > higher framerate.
But then you get potentially physics stuttering or slower animations (think they are tied to the fixed rate somehow).
Then you have to increate the fixed rate again to compensate and the gained fps are lost again:
Normal VaM:
1.0 x 72 Hz fixed rate = 72 fixed rate frames
1.0 x 45 Hz fixed rate = 45 fixed rate frames
"Speedhack":
0.5 x 90 Hz fixed rate = 45 fixed rate frames
0.5 x 72 Hz fixed rate = 36 fixed rate frames
0.5 x 45 Hz fixed rate = 22,5 fixed rate frames
Both green setting should give similar fps then. It's probably cleaner to use / make a plugin to set a custom fixed rate for a similar effect if my theory is correct. Meanwhile the renderer the pushes out the frames at an unlimited speed ... unless maybe if it hits VaM 300 fps limit?
Maybe that's down to 150 with the process time manipulation set to 0.5.
Would be curious what happens if you set it to 0.1 ... if the limit kicks in VaM should be noticeable limited to 0.1 x 300 = 30 fps then.