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

I had issues with the Proton VPN app on my desktop, but my laptop had no problems despite them both using Linux Mint. However, I had no problems with Proton if I set it up via OpenVPN, and Proton has a great guide on doing that.

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.

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.