"Why is p90 wait time on ping thing still so long?"

I think most people (who work on fee markets) understand that wait time in a FIFO m/m/1 queue with excess capacity is long tailed. That is most of the time you arrive when the queue is empty but some small % of the time the queue is full and when its full the wait time is super long in expectation when this happens.

What people might not realize is that this is also the case for priority queues for any given priority level. Supose you are paying a priority fee that is higher than 30% of the other incoming traffic. Then your wait time in the priority queue is at least as long as under the FIFO queue with 70% load, and probably higher and much longer tailed because if you are sitting in the queue, any transactions with a higher priority fee that arrive go before you.