It seems that you're using an outdated browser. Some things may not work as they should (or don't work at all).
We suggest you upgrade newer and better browser like: Chrome, Firefox, Internet Explorer or Opera

Version 4 seems to have helped with the mouse issue. Still rocking 20fps though.
aasgnt: I am also having this issue. No matter the setting the game runs at 20 fps, and refuses to run in windowed mode.

Windows 10 v.1809 (KB4482887)
DevilutionX now renders the game at monitor refresh rate and also allows you to run the game in windowed mode.