Despite it will take still a bit of time to publish CCD, but I'll release
libttk.so much sooner.
It's my retained mode single-threaded terminal UI ...
... i.e, it has jobs but they yield and multi-tasking is co-operative :-) Scenegraphs are thread-safe (or may thread-local?) in the sense that they don't shared local state.
Running fm in single-thread neither does prevent parallel processing thanks to POSIX and non-blocking I/O, and in Linux you have uring.
This is how I look at performance, when I visit the topic. I.e. not thread-bombing but only taking what you need and "stay out of the way bothering" :-)