The goal of these kinds of techniques is to improve performance (i.e., frame rate) by rendering a game at a lower internal resolution and then upscaling that original image to a higher output resolution — without sacrificing image quality too much.
For elements that have sub-pixel detail and change with every frame, like strands of hair, FSR produces a noisy image that compromises the quality of native rendering too much, according to Digital Foundry.
In Performance mode at 4K — which renders at 1080p internally and “visibly impacts image quality,†AMD notes — FSR delivers an average of 2.4 times the frame rate with RX 6000 series cards, according to AMD’s benchmarks.
Depending on your personal taste, that might be enough of a boost to outweigh the Performance mode’s compromises on image quality.