This is a very recent addition in Mupen64plus-Next.
Supposedly, this is an overclock function. This is what i was told by the author when i asked for a proper overclock function like the one in PJ64 and Mupen 64 FZ. But other than that, there is no other information about it. No instructions on how to use it and if any of the other core options are needed to work along with it.
I tried to experiment with it a bit and while i managed to make Goldeneye run smoother for a while, it still slowdowns heavily in many scenes, unlike PJ64 where if you select a 3X overclocking factor (which means the CPU runs 3 times as fast as the real N64) the game runs at 60fps all the time.
Having a proper overclock function for N64 in RA was something i envied ever since it was added in PJ64 (where it works really well) but i don’t think this is nearly as functional. Unless i’m missing something but i can’t find any info.