Lag using Vexnet 2.0 keys but not old black keys

This problem surfaced and we have made no changes to our code. Cortex, joystick and both Vexnet 2.0 keys have the latest firmware. We note a lag to joystick response when using the 2.0 keys. Without re-pairing, we change the 2.0 keys with older(black) keys and the lag goes away. What could cause this?