I’ve observed the problem with other Internet traffic as well. First note I took was actually pacman -Syu being slow. The problem also became evident on speedtest.net.
I found the solution myself just now and feel a bit dumb, frankly. I have several VPN connections that are all inactive by default. NetworkManager was showing no active connection in the GUI. It said “VPN off”.
On the CLI though, I just now found one connection marked as active! As soon as I deactivated it, everything was back to normal.
If anyone has an idea where this drift between NetworkManager’s actual state and GUI comes from, I’m all ears. Maybe it’s just one of NetworkManagers many quirks…
Thanks again for your time and sorry for posting a layer 8 problem.