It should come with a warning label. If you're in a scenario where VRAM is being maxed out and thrashing main memory then ReBAR will worsen the frequency and magnitude of the stutters. I ran into this with a 3070 (8 GB) recently.
It's entirely possible a game manages paging VRAM badly. But allowing the game more flexibility isn't a problem with a larger BAR, it's that the game is stupid and gets dumber the more VRAM you give it.
If a program runs slower when you give it more RAM, the problem isn't giving more RAM.
I think you’re experiencing paging to main memory rather than anything ReBAR related. ReBAR itself reduces memory copy operations. It’s of course possible you’re experiencing some sort of BIOS bug.
