Using the [set drive velocity] and the [set turn velocity] blocks can significantly improve the accuracy of your robot’s autonomous path by fine tuning its rates of travel.
I created a Google doc to help with this, Setting Drive Velocity and Turn Velocity in VEXcode IQ
For more information on this, there is an excellent video, Adjusting Velocity, in the VEX IQ 2nd generation STEM lab - Castle Crasher.