The math work is as follows:
Suppose you earn 100 exp / 100 sp at a 0 gap.
Then with a 3 gap, you earn 70 exp / 130 sp.
--> Resulting in 30% boost in SP per kill, in exchage for 42% slower in leveling rate.
Edited: If you have a main, then perhaps powerleveling your noob at ong ?
Going with 3 gap is not so bad, whereas farming at bandit den is so boring that it makes you wana quit the game
