It's even called "FastUI", that is hilarious.
FastX is popular these days.
The top results I get are for the Fast and Furious movie. After specifying the context ("API", "web", "framework") I get hits for remote desktop software which also has API:
https://www.starnet.com/fastx/ https://www.starnet.com/help/fastx-v3-api/
And "fastx python" returns a module for parsing FASTA files
FastHTTP, FastGPT, FastChat, FastAI, FastLED, FastText, FastClick, FastHub, FastSAM
Most of these have 10s of thousands of stars on github.