Forgive me if I am stating the obvious, but if you are only accelerating out to 20m and holding that speed (especially if you are accelerating at 95% of your max) you shouldn’t be able to get too close to even 90% of your true max speed. I am thinking this workout is one of the CNS depth vs. breadth issues that Charlie talks about.
I do these as 4x4x50m with not much rest early in GPP and fatigue and my coach has always told me that this fatigue is a normal part of the early GPP workouts. We usually rest very little between reps, and a couple of minutes between sets early on. How long are you resting between sets?
I’m sure someone senior will weigh in with the solution.