The approach that helps me the most as I increase the tempo of harmonic slurs is focus on the aperture corners (Shape) and trying to maintain the economy of movement there. Going faster doesn’t require more exaggerated movement or more effort. Rather, trying to minimize the amount of movement and perceived distance between pitches seems to help. The feeling I try for is that of an extended long tone with an aperture corner (Shape) change. Steady relaxed air (passive) also helps.
Getting the system to coordinate as the tempo increases takes a bit of time. I also believe that from time to time it’s worthwhile to push your limits just to take a reading of where you are and where you want to be. However, mastery of the process takes as long as it takes. Patience and a good look with the magnifying glass is often required.
Just my thoughts as I continue to work to improve and become more efficient.