Returnal was built against an NVIDIA SDK old enough that multi-frame generation did not exist yet, and the game hardcodes a request for exactly one generated frame. So 2X is its hard ceiling. I wrote a small proxy that translates the old SDK calls into the modern ones while the game runs, and the ceiling goes away: 3X, 4X, 5X and 6X, whichever you set.
The ceiling was not really in the game. It was in one inverted return value. The old SDK returns a boolean where true means success; the new one returns a code where zero means success. Passing that straight through turns every success into a failure, and the engine then never tags a single resource, so frame generation sits there fully configured and completely idle.
What it buys you is not a bigger number on the counter. At 2X the engine has to render 112 real frames to fill the display; at 4X it renders 56 for the same picture, and the GPU time that frees is what pays for neural rendering. With neural rendering on, 2X gives 130 fps and 4X gives 225. Fewer real frames, 73 percent more of them.
It is free, it is MIT licensed, and it works on Returnal and nothing else, deliberately.