Unity - How do i clamp the speed of my character controller

You should probably find out why, and fix the problem rather than work around the problem.

Did you try debugging? When you dump those values to the screen do they make sense?

I assume you already considered that both x and z have value ‘walkspeed’ when used, so in total, you're moving sqrt(2)*walkspeed. (Instead of 3d think 2d, moving 1 in either x or y has length 1. moving both has length sqrt(2).)

if they are both pressed divide move by 2. Probably the easiest solution.

In 3D, 1+1 vector doesn't necessarily make it a 2 vector. That only works correctly if both vectors are fully aligned. Ie (1, 0, 0) + (-1, 0, 0) is much shorter than length 2.