I seem to have a problem with internet connectivity. I use NetworkManager with Proton VPN on an Arch KDE system. The wifi and VPN stay connected, but every once in a while the upload and download rates will go down to, at most, a few hundred KiB/s. The issue can be solved temporarily by disconnecting and reconnecting the VPN, but as I’ve had a similar issue without the VPN before, I do not know whether this is a VPN issue or a NetworkManager issue.
This kinda sounds like a TCP retransmission issue. Do you have a server available somewhere? Can you run iperf3 in both ways and see the retransmission rate?
You may also want to run TCP with both CUBIC and BBR for this test since that may also isolate shallow buffers versus corrupted packets.