I think you're correct that the wind direction/speed update the most often on most stations.
I notice that with WD clientraw.txt updates, the temperature (humidity, dewpoint) also update more than once a minute with my Vantage Pro.
With the VWS AJAX/Weather Flash, the current data fields update pretty much like the WD clientraw.txt fields update .. the running average fields (and sometimes the hi/low/rate/rate-min/rate-max values) change with almost every update as they are recalculated.
With a non-VP station, the data values may be much slower in change, depending (as you say) on the update interval of the weather station itself.
Best regards,
Ken