Speedy harmonic slurs (fifthts) are also my problem. Although I focus on this issue since about a year, I seem to stuck.
When I heard Greg’s secret “your lips are too strong allready…” I had to smile, because my meaning was exactly the opposite!
Meanwhile I consider he is right…
A lot of muscle action is going on if I activate my aperture corners so my mouthpiece is slightly moving around also the lower lip and when I accelerating, the movings get stronger, the sound instable.

Now I’m trying to do slurs with the lead pipe sustaining the airflow and moving the aperture corners slightly, try to relax, minimise all movements and play with passive reduction.
Nevertheless the speed as demonstrated by Greg seems to be from another star for me!