There is definitely a balance to power and speed; if you run too slow or with too much power, you can discolor the stainless steel. I highly recommend having a sacrificial cup or piece of metal around to test settings on. As seen in the image below, I selected 3000 mm/s with my power at 60% and frequency at 65KHz. My settings allowed for a nice dark engraved mark without overheating the metal surrounding the engraving. If the metal does overheat, one of the first signs is a tan almost roasted marshmallow appearance on the metal around the engraving.
Image courtesy Braden Todd.