Not that good, if you use the built-in 1Hz GPS.

If you use an external 10Hz+ GPS then it should be good.

Hi!

The 0-X timers use the most accurate available source, so if you have no OBD2, then it defaults to the GPS

You can also augment the GPS to use a 10Hz (or 20Hz racelogic) GPS device, which will improve things

If you are using OBD however, then it will automagically default to using the OBDs sensors for measuring speed (the GPS is then used for wheelspin detection to make sure the readings are valid). Typically the OBD2 speed measurement is extremely accurate (unless you have changed the wheel diameters, in which case your speedometer will also need adjustment).
If you are using OBD2 over CANBUS, and only have the 0-X speed sensors visible with no extra logging enabled, then this can be as fast as 100Hz updates on some vehicles (basically dependent on the sensor read speed), and is preferred over the 'slow' GPS method