Hm, strange, since the last 2 or 3 versions of sdl I recognized the same problems with Ermentrud.
I compiled Ermentrud with new versions of SDL2 and in fullscreen mode everything is totally
slow an d eats 100% of the cpu. No matter if I use software or ogl rendering.
In window mode everything is ok.
I wanted to report this a while ago, but did not find the time yet or investigate this a little bit more.

But good news is that Ermentrud works with ogl now on mos.