With a mecanum drivetrain is driving sideways (along the Y axis below), there are four force vectors (one from each wheel) directed at 45 degree angles:
The Y components of the force vectors make the robot move in the Y direction, and are each equal to sin(45)*F=0.707*F. The X components, which are also equal to .707*F, cancel each other out.
Therefore, the robot will have .707 the force driving its sideways movement as it will driving its forward movement.