I've been using Mullvad for going on 4 years now, but I've also used Proton VPN. I recently switched back to Proton a few months ago after realizing that I had credits that I completely forgot about.
One thing that I've noticed is that Proton seems to struggle with maintaining a consistent connection while I am away from home and have it activated (over public Wi-Fi networks). I've seen it constantly try to reconnect. It's rare for me to experience that problem while using Mullvad.
I have also noticed that streaming videos can be a pain for me when using Proton. Whether at home, and streaming with Google TV boxes, or tablets, videos will stop quite a bit to buffer. And that's while trying to stream 720p. Again, I don't have this issue will Mullvad. My Internet is pretty fast, especially when I have VPN services turned off.