Why is downloading so horrendous

Every single time I update games in the blizzard launcher I usually go from 25mb/s to a couple of bytes per second. I’ve been troubleshooting and followed every step from there but yet I end up in this slow downloading process and I don’t want to wait a week to be able to play. Is there an explination why the downloading is so horrendously slow?

The 1.22 patch is going to have periods where the download appears slow but that is because it is actually installing content before it resumes downloading. This is for this patch only and will not likely happen in future patches.