DAQmx+6009: Reading the current counter value is too slow

Question

I have a thread that runs a software loop that I need to run as fast as possible. One of the things I need to do is poll the current counter value from my 6009, and branch if it's changed from the last time I read it. Over 90% of the time spent in my 1.2ms loop is doing this read. My goal is to get the loop under 50us.
What I really need here is an accurate timestamp (20us or better) for every time the counter changes (up to 1kHz). When I detect a change, I currently get the timestamp by calling QueryPerformanceCounter().
Here's how I create the task:
    DAQmxErrChk( DAQmxCreateTask("",&taskHandle) );
    DAQmxErrChk( DAQmxCreateCICountEdgesChan( taskHandle, line, "", DAQmx_Val_Falling, 0, DAQmx_Val_CountUp ) );
    DAQmxErrChk( DAQmxStartTask(taskHandle) );
And here's the line of code that goes so slow in my loop:
    DAQmxErrChk( DAQmxReadCounterScalarU32( taskHandle, 10.0, &count, NULL ) );

Answer 1

I updated a robot control from a Mac Powerbook with a PCI expansion chasis running Mac OS9, Labview 5 and DAQ to a Mac Mini (intell) with 6009 and 6501 USB cards  running Mac OSX, Labview 8.5, and DAQmx-base.  I have connected wheel encoders (pule train and direction) to a 6009 and a 6501.  It all seems to work, except the tuning of velocity control loops is not as good as before, indicating some delays.  The loops run every 100msec (10Hz update) so I expected a synchronisation problem rather than latency.  I did some measurements today and found that reading the 2 counters and the 2 digital inputs takes over 20msec.
 I get better performance from VISA with a serial to USB converter.  I have a program reading 25byte packets every 20msec.  Although I did run into the USB latency problem with another serial application.  I have a serial device that requires a response t changing CTS (clear to send) in less than 1.2msec, and VISA was taking 1.5msec.
On the same machine I am reading 1024 samples from an analog input every 100msec with out any timing issues - (once I got the incredibly slow start and stop vis outside the loop - they take about 200msec each.  Taking the start and stop vis outside the loop reduced my loop time from 450msec (approx) to 100msec.  The only problem is that to do this I have to run in continuous mode and can't  use an external trigger (it has to restart on every trigger, so the start and stop vis have to be in the loop).
So it appears that I have updated my technology by 10 years only to find that USB is incredibly slow - something that is not in any of the NI doccumentation that I can find.  Does NI have any documentation on what to expect in terms of performance from these USB devices and any suggestions on how to get better performance, apart from buying more expensive hardware?
Thanks