I agree with the OP.
The difference between 30 and 31 is very daunting, thus, creating a lack of motivation, therefore, a longer time at 30.
I would suggest that the difference in exp could be lessened by some 500k-1m, as the 29-30 gap is quite minor in contrast to the 30-31 'grind'.
A relevant metaphor could be perhaps, "level 30 to 31 is like being thrown in the deep-end", as 29-30 is <2m exp, and once they hit 30, they need around 4.5m in order to hit 31.