Here, this is the most important graph to understand X-Drive:
Source: AURA
If you drive forward, backward, or to the side - all four motors are providing power, the velocity is MotorRPM x sqrt(2) and pushing force is 4 x OneMotorTorque/sqrt(2).
If you drive at a diagonal, then only two motors are providing power, velocity is MotorRPM and pushing force is 2 x OneMotorTorque.
Essentially, X-Drive speed is 1.41 times faster and pushing force is 1.41 times weaker than the ordinary tank drive with the identical 4 wheels & 4 motors.
Also, here are the links to the similar topics from the previous seasons: Two motor drives?, X-Drive Speed, Extinct X-Drive?