They also probably mean TUIs, as CLIs don't do the whole "Draw every X" thing (and usually aren't interactive), that's basically what sets them apart from CLIs.
It’s surprising how quickly the bottleneck starts to become python itself in any nontrivial application, unless you’re very careful to write a thin layer that mostly shells out to C modules.
You probably mean GIL, as python has supported multi threading for like 20 years.
Idk if ranger is slow because it is written in python. Probably it is the specific implementation.