I just installed apt cacher ng for catching my apt upgrade packages and saw a huge time improvement even though I have a good internet connection. It act as a proxy and caches the response packages.
Do you run something similar? Or maybe even run a local repo mirror? Warning, they are really big so I don't think it is recommended unless you really need almost everything.
I use it with Kubuntu. Doing apt update is now much faster. I did some testing and found some good public mirror so I could max my connection(100 Mbit) with about 15ms latency to the server. But I think the problem was there are so many small files. Running nala to fetch the files in parallel helps of course. With apt local ng I don’t need nala at all. The low latency and files on gigabit connection to my server leads to fast access. Just need to find a good way to fill it with new updates.
A second problem is to figure out if something can be done to speed up the apt upgrade, which I guess is not possible. Workaround with snapshots and send diff does not sound efficient either, even on older hardware.
apt update - 4 seconds vs 16 seconds.
apt upgrade --download-only - 10 seconds vs 84 seconds;