Thanks for the reports! Always great to get some feedback from a linux user ^^
I'll work on the menu bug, that happened to me recently too.
I think the weird colors are intentional-- we've had 'elite' monsters in for a while and they always have a unique color scheme. Maybe that's not what you are talking about though.
Yet, when I go "outwards", the game does slow down for a couple seconds after some distance.
So if I understand correctly, the framerate decreases for a few seconds and then normalizes? Is it one continuous slowdown, or several fairly short slowdowns in the space of a few seconds?