I had the same experience a few months ago on all my devices at home including firestick 4k! I ran all sorts of Network tests you can imagine, and all were unsuccessful. I even emailed Proton at the time and they basically said unplug/replug 😂.

Street a while, I fixed it by disabling VPN accelerator and got my usual speeds.. I suspect that may have fixed the issue then or it could have been my ISP to blame.

The only reason I'm not using it all the time on my GOS now is because of my node config.