For my 60 W laser, the offset correction became significant over 100 mm/s, so it’s not surprising (to me, anyhow) that you don’t need it below 150 mm/s.
The offset timing depends on the HV power supply timing, so it will be different with the 90 W supply. Measure and correct as needed!
That looks like a rather orderly version of the random dots I explored:
They’re probably on the tail end of the engraved line, where the tube has been lasing and the HV power supply can’t tamp down the tube’s conduction. You may get fewer dots when you run at higher speeds with higher powers.
If they happen with regularity on every engraved line, no matter what power you use, a warranty replacement seems appropriate. Definitely let us know how that works out!