Hi,
I have a looked at megaparsec library it is using nix develop for setting build environment and nix-shell, I used to, is not working.
nix-shell is very talkative and prints lots of information. This is helpful especially during first run when local cache is empty.
nix develop takes long time for first run and CPU usage is high, but terminal is quite.
nix develop --help doesn’t mention anything like --verbose or some log option.