data sampling frequency of thingworx platform

I think there's only soft limit over the sampling speed. 

Note that all DataTable, Stream and Value Stream have their own and only table in persistence provider, so the number of properties being updated and the frequency of update both counts, and the total write frequency limit from database side would affect this performance.

There are sizing guide for running TWX server with average suggestions, but they cannot give you the upper limit. Only thing that's confirmed is that ValueStream has lower priority than DT and Stream data, some changes will be ignored if the server is over the capacity.