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.
You must isolate the problem to figure it out.
Try with another device using the VPN and try with your computer without the VPN.
Also I would suggest to check if this isn't related to your wifi or router configurations/setup. Once I had a router that sucks a lot and for once in a while I had to reconnect or even restart the router to make it work again
All of my other devices are either Windows or Android for convenience. The VPN works on Windows, but I can't isolate this issue via that route as those devices are old and have their own issues.
I have tried running my system without vpn and this specific issue hasn't appeared from yesterday until now. It is making me feel a little uncomfortable not having it on though.
I'm going to try another suggestion, then I'll come back to this one if it still doesn't work.
Proton on Linux is annoying to say the least. I've got it mostly working on my distro and without a DE. Have you checked the protovnpvn_reconnector service, assuming you're on systemd? It's kind of odd that it just slows to a crawl but maintains connection.