It's always dependent on the time of day, as the more folks in your local area connecting to the local node will slow down everyones speed. Are you using a free VPN or do you pay for the service? This can drastically impact the speed as the number of Proton users on the server you're using also can reduce your speed.
Alternatively, do you use a DNS to filter BS connections only there for tracking? Paying for the VPN + using NextDNS to stop unwanted connections has resulted with my speeds being a minimum of 3MB with a max around 150MB. I have a 300MB connection, but the time of day, DNS filtering, as well as routing through a VPN all influence your speed. Thankfully these speeds are more than enough for my use case and also add to my ability to have online anonymity.
Edit: I use the official client but use the WiFi settings in Fedora to connect to my DNS server. There's also a setting to increase speeds in the Proton client, maybe give that a shot too if you don't have it turned on.