XferCommands are confusing to write. I have one, and it makes my downloads faster, though I wouldn't have a clue how to write it myself. Here's what I have currently:

    XferCommand = /usr/bin/printf 'Downloading ' && echo %u | awk -F/ '{printf $NF}' && printf '...' && /usr/bin/aria2c -q --allow-overwrite=true -c --file-allocation=none --log-level=error -m2 --max-connection-per-server=2 --max-file-not-found=5 --min-split-size=5M --no-conf --remote-time=true --summary-interval=0 -t5 -d / -o %o %u && echo ' Complete!'

It's fast, but doesn't have download progress. I had another one, which printed several lines of output per file (but did have progress!).

My point is, the fact that you can make pacman download stuff quickly as-is, doesn't mean that inbuilt support wouldn't be beneficial for everyone who doesn't know how their downloading program works back-to-front.

iirc, apt has support for it (it will only download a single thing at a time from a given host, but does appear to download different things from different hosts at the same time).

I believe gentoo's emerge also has support (though I'm not as sure here as I typically have the downloads happen in the background while building).

The overall goal here is to more effectively utilize the available bandwidth on the client side even when there are limitations a given server.

It's great that we can customize how to download 1 package at a time. But there is room for improvement.

There are multiple ways to do it, but the defaults matter.