So, see, they didn't say anything about speeding it up for 18-20th or one rate in tier-2 thru 9, higher at 10 only, then faster for 11 to... etc etc etc.
Seems like to them, those designers folks, they seemed to think what mattered, what the expectation was, was little more than " move thru the first couple levels in just a handful of sessions to get to the rest of the game which has a somewhat slower pace for the rest of the campaign."
Mike Mearls did say, however, that the speed-up at level 11+ was intentional.