That depends on how the motor has been timed.

When using an electric motor for applications such as driving a propellor or the wheels of a car it is normal to time them so they're most powerful/fast when rotating in the desired direction.

However, when using them in an application that requires them to operate in both directions (as is the case with an RC servo) the timing should be set up so that it's neutral for two reasons:

1. the speed will be equal in both directions
2. the torque will be equal in both directions

I've yet to see a servo (Hitec or otherwise) where the engine wasn't set up with neutral timing and as a result I've never observed the difference in speed you talk about.