Apparently, on RDNA3 RX 7900XTX, this INT8 version of FSR4 uses 1.9ms per frame vs 0.6ms for FSR3. Not sure at what quality settings, I'm assuming Performance. What I've found is that FSR4 at same settings on RDNA4 RX 9070XT uses ~0.5ms per frame, with FSR3 ~0.2ms.
So, while there is significant hit per frame, the thing with upscaling is the higher your initial frame rate is, the less of the impact for final frame rate it has.
Found this (don't know how to embed Reddit videos):
Of course, RX 7900XTX is top of the line RDNA3, which is why Steam Deck with it's low end RDNA2 GPU is having up to 20-25% hit on fps, but I'm thinking now that the cat is out of the bag, all other AMD based handhelds will see this feature implemented, possibly even officially.
