Edmund,
The digital lines on the DAQCard-1200 do not have hardware timing associated with them, so the maximum read/write rate will depend on software timing.
This means that the read/write will depend on how fast your code can execute. One thing to remember is that as other programs call for CPU time you application code will slow down and if nothing else is using CPU time your application will run faster up to a point.
If you need hardware timed digital I/O I would recommend using a DAQCard-6533.
