AFAIK the virtual position is updated every 2 ms, so at a rate of ~500 Hz (± some jitter due to other aspects in the control loop and such). You’ll get a lower rate than that since you have to add in the time required to read the stuff from the LSS and process it (+ delays & jitters from UART/USB comms).
You may want to look at what @cmackenzie (posts here, you’ll have to search in there) has done with his code to do fast control of a full humanoid.
You can find some info in this topic.
And, to directly control the motor (skipping its internal control system) you can use RDM. See the details here. Warning: by using RDM it is easy to send a signal that causes power to be used but no motion (i.e.: the signal is too low for the amount of torque required due to load) and therefore overheat and possibly damage the motor if kept going for too long in a “stuck” state. Be careful!