DAQmxReadCounterF64 execution time

Hi, all,

I am trying to test the example code at

Examples\DAQmx ANSI C\Counter\Measure Digital Frequency\Dig Freq-Buff-Cont-Large Range 2 Ctr

I changed the original code 

DAQmxErrChk (DAQmxCreateCIFreqChan(taskHandle,"Dev1/ctr0","",100000,1000000,DAQmx_Val_Hz,DAQmx_Val_Rising,DAQmx_Val_LargeRng2Ctr,0.001,10,""));

to 

DAQmxErrChk (DAQmxCreateCIFreqChan(taskHandle,"Dev1/ctr0","",100000,1000000,DAQmx_Val_Hz,DAQmx_Val_Rising,DAQmx_Val_HighFreq2Ctr,0.001,10,""));

I found that, the time to execute the following command is very slow (~8 second) if my input signal frequency is low (~20 Hz).

DAQmxErrChk(DAQmxReadCounterF64(taskHandle, 1000, 10.0, data, 1000, &read, 0));

Since I use the 0.001 second in the measTime, I supposed it should be very fast. It should be around 1000 * 0.001 s = 1 second.

It behavior normal if my input signal frequency is high.

Thanks for help from anyone first!