There are definitely faster actuators but I'd guess it's also the control loop(s) filtering out transients from the stepping motion. The backlash in the geartrain, flex in the body and gritty/chunky motion from the servos probably makes a mess in a typical six axis imu.
I guess what is "fast" though, I suppose it is not impossible for this particular design(that looks like it would wiggle/swivel?) due to the horizontal moving main joints. But I have not done calculations to prove that(which would be faster horizontal or vertical swing)
edit: which is interesting arguable horizontal join/swing design is less work
