It has been my experience that continuous rotation servos frequently have poor speed control.
The control hardware and firmware is designed with the positionally aware servo design in mind. Given a particular position command, go there as quickly as possible and hold that position.
When this hardware/firmware is re-purposed to be a continuous rotation servo, it does one thing very well, let you command the motor to rotate forward, backward and stop.
But it often does NOT give usable speed control.