The issue is that the motor is that the torque developed by the motor is only marginally able to accelerate the higher mass of the metal CBN wheel. Motor torque is closely related to its horsepower rating.
If you've shopped for a lawnmower or snowblower in the US recently, you may have noticed that they are rated in terms of engine displacement rather than horsepower. The reason for this is that manufacturers were playing games with the way the determined the horsepower ratings, with the result that consumers were being mislead into believing that they were purchasing a machine that was more powerful than it actually was. Changing the ratings to engine displacement rather than horsepower forced manufacturers to use a parameter that was less susceptible to manipulation and misrepresentation.
A similar thing took place in the high-end sound system business many years ago - manufacturers were manipulating the power rating assigned to speakers and amplifiers.
It would not surprise me to learn that if one were to actually test a brand-X Chinese motor sold by HF with a "3/4 horsepower" rating in accordance with NEMA MG-2 standard for motors, the results might show that the true rating is less than 3/4 horsepower.