Did as you proposed. With nightly build, your changes to:device.lua&
Koreader is slower (2-3times) in the menus, pageturning is fast and it felt like it took more time to crash it. File opening feels like 20 times slower.
Now that is strange: I enabled nightmode and everything is slowed down so much it feels like I'm waiting more than 2-3 minutes for any reaction after my input. After disabling Night mode everything goes soooo fast.
Edit:
Thinking about it I guess Koreader didn't even crash, just slowed down soo much (nightmode) that I thought it crashed.