Changing CV5 to 0 is fine but that takes the top speed voltage cap off of the CV. Next time put it on the Programming track and read what is in CV5. Then you can adjust it up to raise the top speed. This CV controls the voltage to the motor at the full top throttle speed step. Putting a value of 0 just nullifies this CV.
If you happy with 35-40mph as a top speed, then put it on the Program track and read what is in CV6 the mid voltage CV. You can then go to CV5 and add 50% of the value of CV6 into it. This will give you a top speed in CV5 and make it functional again. If the value you put in still seems too slow you can add to it in the POM or ops mode to get the top speed you want. Try that and see what happens. I put a top speed on all of my engines using CV5 because they all run to fast at the top speed step.
My choice for switcher is no more that 25 mph, my road swtichers and freight engines no more than 40-45 mph and my passenger engines 80 mph. I use CV2, CV6 and CV5 to adjust this.