@chownki it's because the inductance and back emf of those motors exceeds the power supply voltage at those speeds. When this happens, the motor positioning becomes less accurate, and the motor gets noisier.
If you are using 12V power in your machine then upgrading to 24V will allow higher speeds, although of course this normally requires different the heaters and fans.
Use our motor calculator at https://www.reprapfirmware.org/emf.html to estimate the maximum speed before the supply voltage becomes insufficient.
