The new one is at like 5fps and the old one is much faster. Does it render all mobs at once when in the menu? This does not affect the "edit" mode (60fps) A solution would be to possibly render only the mobs on screen?

I suppose it actually does do that... I'll fix that sometime soon. 😓

It took quite a while to fix this, but the fix will be available in the next update. Thanks for reporting!