Not uncommon, especially when reaching top speed as you mentioned. If you are doing A, then low C it’s usually a matter of crunching the trigger due to speed and not getting a good pull. If you are going low C, low C or low C and then D or M, then it’s an aiming issue with the dot and then some trigger crunch. My suggestions: try shooting 5 or 6 shots as fast as you can on one target and do it until mostly As or a C or two to the side (but not low). This will force following the dot and making sure you have a very tight grip (you said you are not new to the sport, but when we go at high speeds the grip sometimes loosens up to be able to manipulate the trigger faster. Stay tight with the support hand, keep the strong hand more relaxed). Once the hits are good on this drill, then work in another target to transition to. So, 2-3 rapid shots on one target and then transition to the second and do the same. This will tell you in you need to get a better sight picture on the first target or the transition is what’s tripping you up.
Now, it “could” be a timing issue based on your load and spring, but 8-10lb spring with that PF should not cause low hits. However, every open gun is a bit different.
I shoot open, 9mm Major, customer shorty (4.25in barrel) 2011 with 6 ports and 10lb variable recoil spring. Load is 124g JHP over 7.0gr of WAC with OAL of 1.168-1.170. PF is 171.