Edgar, the servo spends most of its working life "needing" an update and very little of its time getting an update. The servo data pulse will be between 0.9 and 2.1ms long. Then it has to wait approx 18ms with no data until another data pulse of 0.9-2.1ms comes along and then it sits idle for another approx 18ms. Thus changing the refresh rate from 21 ms to 18ms simply means it sits idle for 3ms less but it is still sitting idle for approx 15ms. It makes no practical difference, the servo electronics are designed for a pulse at around 20ms but with latitude either side of that.
Servo behaviour like that is much more suggestive of a faulty servo. You used the magic word "Hitec" and that's the first place I would be looking for any fault. I won't let them into my workshop let alone into my models. But that's a story for another thread.