As far as I know, coroutines are there to run code without stopping the thread it is currently residing in. They aren’t meant to run exactly at the same time, only running alongside the current thread.
ROBLOX has been testing a new feature called Parallel Lua which I’m sure runs Parallel to your thread which may be what you’re looking for: Parallel Lua Beta. Sadly, it is still in heavy beta and is not allowed outside of studio.