Honestly the GTA5 downloader/updater itself has pretty bad configuration. I wrote a post about it on Reddit years ago along with how to fix it.

I don't know if it's still applicable or not because I haven't played it for ages, but just in case it is, here's the post: https://www.reddit.com/r/GTAV/comments/3ysv1d/pc_slow_rsc_au...