Exp rates are fine i got level 55 in around 10 days. You are encouraged to autoparty as it gives 2% xp bonus for each member in your party. The grind from level 40 isn't that bad not when you compare the grind to level 52-55. The main quests should give you a huge chunk of your exp.
The way people are level 55 already is because they are MAT hunting and doing suicide runs where only half your energy is taken
and also MANIC mission 4 only. Manic mission 4 gives more xp than mission 3 2 1 of the next city.
Ahh if it isnt Napoleon lml mats for days in the forttess than burn it in manic the grind isnt that bad but its still there. My only true issue is its not the same xp rate across the board