Index update slows down when screen is turned off
There is already #563 (closed), but this is somehow different:
The time while updating the index when the screen is turned off increased a lot since it went into a notification, IIRC. Maybe someone can record the log for an index update with screen turned on and once with screen turned off.
"[M]any people have reported it taking up to an hour if the screen is off", as @mvdan stated on IRC. For reference: The discussion on IRC