It's on developer's site, just a little more of search than usual http://sk8tokyo.com/udder/prog/xop/index.html http://sk8tokyo.com/udder/prog/xopblack/index.html

Other game versions and media are linked on the Discord server

1 year later I reply to you, there is 1 way to move faster, and it's with weapon knockback. I did measurements, that prove, duck travels faster when shooting+sliding. But is it worth it? Depends on weapon, higher knockback = faster traveling.

Also some mods add items that increase duck's overall speed