Yeah, doing Usopp's quest will consume a lot of time because even if you play the level at 3x stamina you're just spending 15. Doffy is very fast because you spend 180 stamina doing the quest at 3x stamina.
I completely forgot, but we'll get soon another legend Marco that will boost EXP by 2,25, while Jack boosts EXP by 2x. Obviously, don't pull for him because anni is in 2 weeks, but wait for his release so you can use him as Friend Captain. It's just a 0,25x difference but in the long run it's a huge amount of EXP you're gaining.
Anni ship boost (3x) x Jack FC (2x) x Jack as captain (2x) = 12x EXP boost x 10.000 EXP = 120.000 EXP
Anni ship boost (3x) x FC Marco (2.25x) x Jack as captain (2x) = 13,5x EXP boost x 10.000 EXP = 135.000
Anni ship boost (3x) x FC Marco (2.25x) x Marco as captain (2x) = 15,1875x EXP boost x 10.000 EXP = 151.875 EXP
So, let's say you play a quest that gives 10.000 EXP if you win. With just 1 Marco you get 15.000 more EXP per run, and if you play that level 100 times you get a total of 1.5M more EXP. Jack is already great but a FC Marco definitely helps.