Rainbow Six Siege Insane Load times? Computer or Internet?

I think the real bottleneck here is your hard drive and/or your internet connection. Seeing your specs, your CPU is way better than mine, and so is your GPU. Yet, your loading time is 1-5 minutes, while I'm never above 30s.

I advise you buy an SSD and store your games on it, the loading times are amazing! A 4TB one would be pretty expensive, but do you really need this much storage? The usual compromise is system+games on SSD, music/video/pictures/etc on hdd. Moreover, if you're using steam/uplay/origin/whichever one I forgot, you can install/uninstall games without loosing progression. I really doubt you're playing 4TB of games simultaneously, so a 500/750 Go SSD should be enough, and it should greatly decrease your loading time.

For me, it was HD Texture Pack DLC which increased load times dramatically. Try uninstalling it.

Alternatively, you can try moving game textures to some kind of RAM Drive which in theory should help with increasing speed of HDD reads.