Typically speeds when working with diode lasers are expressed in terms of mm/minute, rather than mm/second. When you are comparing speeds did you factor in that difference?
Some level of power drop-off is typical with use/age of diode. However, a dramatic drop-off may be an indicator that the diode is on its way out. I suspect the life on yours may have been shortened due to consistent and prolonged duty at 100% power. How many hours of usage do you think you have on the laser at 100% power?
Heat is the real killer so if you’ve pulled power on the fan immediately after completing a burn instead of letting the fan turn off on its own can accelerate degradation.
This covers a maintenance item that would be a typical cause of power reduction.
Also, you may want to make sure that the laser focus hasn’t changed in some way.
One other thing to check is that “S Value Max” in Device Settings is configured to 1000 to match $30.
Your laser should be warranted for a year in the case the diode is indeed giving up the ghost.
You can read more about the controversy around running at 100% duty here: