The fact that your “rant” is directed at Git is misinformed, because it is very unlikely to be related to the issues you are experiencing.
No issues here.
==> Retrieving sources...
-> Cloning mpv git repo...
Cloning into bare repository '/home/jeremy/.cache/paru/clone/mpv-git/mpv'...
remote: Enumerating objects: 341779, done.
remote: Counting objects: 100% (12940/12940), done.
remote: Compressing objects: 100% (1481/1481), done.
remote: Total 341779 (delta 12224), reused 11642 (delta 11453), pack-reused 328839
Receiving objects: 100% (341779/341779), 114.77 MiB | 4.14 MiB/s, done.
Resolving deltas: 100% (269003/269003), done.
I just installed this package with Paru to test and it took about three minutes to download, build, and install.
It sounds like you have some network configuration issues.
When you build a package from source, it needs to pull down all of the libraries and dependencies necessary for building the package. If you want a binary, just install the normal package with Pacman.
That seems unnecessary. I’m on a work machine that only gets 30 Mbps down and I can still clone this repo in about thirty seconds.
Again, it seems like you have an issue with your network.
If that were true, this post never would have been written. 
We can already tell, based on your ability to use Git. 
Your issue is not Git, it is your network. Do you manage the network you are on or are you connected to someone else’s network? If someone else’s, it’s possible certain kinds of network traffic are being intentionally throttled.