Try running the workout with EBC on your phone next time.
If no lag on EBC the issue is with how Garmin is processing the telemetry data from the Connect IQ app via Bluetooth to Garmin Connect on your phone which in turn passes the data on to XO. Try disabling Wi-Fi on the Garmin to see if that make a difference.
If the same lag occurs with EBC there is an issue with XO server connection to your location.