max_velocity in mm/s is the maximum speed Klipper allows the print-head to move. The higher the faster max_accel in mm/s^2 is the maximum acceleration Klipper allows the print-head to move. The higher the faster.
The actual speed is determined by your slicer settings up to the above maxima.
Typically higher acceleration has a bigger impact than only the velocity setting. What can be achieved mainly depends on your printer’s mechanical capabilities and also the speed of your board / stepper settings