If the both Decentraleyes and Dark Reader are enabled, and while certain pages are open - one such being Protonmail - the browser freezes, it takes more time to respond to user input (mouse interaction, keypresses and keycombinations including Ctrl+W), and if you don't close the problematic tab in about 30-60 seconds then the browser will respond so slow that you'll need to kill it's processes to fix it.

After testing in a new profile without any kind of settings and only the 2 plugins installed it seems it depends on one more thing. I'll try to find it now, and write about my findings in a following comment Settings are unaltered of both plugins in both the test and main profiles, so plugin configuration can't be the problem.