This question is asked many times on the forum and has not been answered. Can we get unity staff to weigh in on this please? For me I am attempting to change the run animation clip of the third person beta asset controller. The z velocity of ANY run animation I have imported is 0. The Run animation included with the controller has a positive Z value. No run animation (or other setting for that matter) replacing the OEM run animation works for me. The character simply stops moving forward and runs in place. Put the original run animation back in the character runs again with positive movement on the z axis forward.
in Unity a rigged character animation could be rooted or not rooted indeed, and the unity Animator component have an option to choose that. If you have a rooted animation - as is it moves along the z axis - and you check Apply root Motion in the inspector, then the animator will take care of the character z translation, otherwise, for animation not moving along z axis, it’s up to you using the movement methods of the transform object. So, to answer to your question “how to change the average velocity of an imported animation”, be aware that you can change the animation velocity only for rigged animations having root motion and you do that using the state-machine animator window, but to know how, you need to read this section of the manual and find out by yourself 'cos it is too long to explain here.
An easy workaround fix is just change the Animator.Root T curve in the Animation window of the specified clip. If your using a clip that’s a read only just duplicate and rename it. Than delete all the needless keys between your start and end frame - make all the tangents linear - change your end Z key frame for that clip - apply Root motion in the Animator component and your model should start translating on the Z-axis.