If it's a 2 in 1 deck I think that means it may be 52", as the smaller decks aren't convertible, I think. 52" with the MB is a lot of deck if it's just the 18hp engine choices. Not a lot of torque there. Definitely try slowing down and cutting less off. There are limits to mulching even with plenty of power. You may have to make a second clean-up pass many times, though that's still probably not that much worse than bagging in the big picture.