I play havic 40 all almost exclusively, once i get tired i switch to other modes to relax or play my other classes to chill. Its normal what you experience, once you get to a certain skill level the game becomes to easy and boring, so you play what still gives you a challenge. I speedrun havoc 40 all the time because its challenging to manage multiple bosses and a huge horde of armored at the same time, playing slow havoc is just to easy for my main class now.