Why FPS matters when comparing upscaling quality

I posted a small snippet of information on bluesky recently about reviewers comparing upscalers. I have seen outlets compare images rendered with GPUs in different market segments, such as AMD Radeon RX 9070 XT to Nvidia Geforce RTX 5090. I stated if this is done, FPS needs to be limited to be a fair comparison of upscale quality - this post explains the technical reason as to why.