Do you really spend 18-24 minutes on the ground due congestion? If that was me as a student I would be seriously considering another school/aerodrome as that is clearly a busy show.
Re paying the instructor... the instructor logs instruction time from time the aircraft moves until it stops at the conclusion of the flight (and you log that time in your log book as flight time).
You seem to be saying you should get that time for nothing because the instructor is not effectively working.
Even on the ground the instructor is in command of the aircraft and hopefully from their employers and customers perspective, actively involved in the flight and lesson's progress.
If the student stuffs up on the ground (and it can happen) - clips another aircraft's wing, misunderstands an instruction from ATC and infringes a clearance limit, takes the wrong taxiway, misses a radio call, damages the aircraft from mishandling, doesn't detect a fault, flicks the mags off in run up then flicks them back on a second or two later with a loud bang, misuses the radio and clogs up a busy situation, misses completing a pre-take off action or sets the aircraft up for flight incorrectly or whatever... the instructor is the one who wears the blame. He or she is in command of the aircraft and should not allow it to happen and is the one liable. Are they really not being effective? Would you take that responsibility without pay?
I work out of a moderately busy class D aerodrome and I average 0.2 (12 minutes) per flight for ab initio students on the ground. Early ones are longer mainly because they are slow and wobbly doing checks, make mistakes and are clumsy and nervous with RT and need time to practice and discuss what is happening but they usually speed up to average a little over 6 minutes out a little under 6 in.
That time is not wasted as far as I am concerned, it involves me monitoring them safely preparing the aircraft for flight, safely and appropriately handling the aircraft on the ground, correctly interacting with ATC, polishing their R/T, building up their situational awareness and actively teaching them things and reviewing their performance and looking out for gotchas (some that could strike after take-off if I wasn't on the ball on the ground).
I think I deserve to paid for that.