Hi Rob,
More thoughts.....
Since, if I'm not mistaken, the time is parsed from the NMEA string using the PPS signal to initiate a display update will update the display with the time information from the last received string which is basically one second "old". I think that the best you can do really is to simply grab and parse the string and then update the display.

If you're receiving the full 82byte NMEA string that will take ~170ms, add the actual display update code to that - you'll be a couple of hundred ms off.

/Henrik.