A regular servo converted to continuous rotation will give you around 60-70 RPM. That is a bit slow for a speed competing robot. You may find faster servos that will raise the RPM to 80 or a bit above, but still under 100 RPM. For a speedy robot, you need to raise the RPM around 200 or more. If you use PWM, it is better to have a higher RPM motor to have enough speed when needed, then slow it down for regular use. I wanted to get a a motor with built in encoders with around 200 RPM at a reasonable price for a hobbyst, but what I found available is not much faster than a fast servo (80 RPM), so, to increase the speed I need to use big diameter wheels, like Admin is using on his ERP robot. At 80 RPM (at 6V), using the Lego motorcycle wheels, I get 34 cm per second. If I will also raise the voltage to 12V (120 RPM), I will get 51 cm per second which is fine for a house roaming robot.