My brother bought the game a few weeks ago and has levelled up all the characters. Two of the characters during the XP bonus event. He thought that after level 20 it became very slow and unrewarding. Between 30-35 it was painful because he could need 3 games just to get one level. We played Legendary all books & some Chaos Waste. During the event with the XP bonus he received one level per game after level 20ish and it was much more enjoyable for him.
I think Fatshark should increase the XP permanently, at least for the last 15 levels. It would be easier to convince people to play the game and it would be a more pleasant experience for people levelling.