Common Lisp is not terribly slow to start, and people rely on its REPL when programming in it. Even Python's REPL is quite popular, and it's not slow to start either.
The REPL is relied on because it's useful for exploring parts of a system or program. It's like being able to both dissect and vivisect the system without having to go through all the normal issues of setting up a debugger and instrumentation. The environment and program is just there.
I never really did much REPL programming until I started using pdbpp with a bunch of aliases and customizations in pdbrc and pdbrc.py (the former does aliasing[0] the latter does setup before boot) it makes iterating with a test suite so much faster.
[0] For example:
alias cc import pdb; pdb.disable()
alias ppr pp response.json()
The REPL is relied on because it's useful for exploring parts of a system or program. It's like being able to both dissect and vivisect the system without having to go through all the normal issues of setting up a debugger and instrumentation. The environment and program is just there.