NI DAQ counter rate is slow with Matlab

Hey,

I have NI 6602 and NI 6259 for counting, and I found out the counting rate I can achieve is a lot slower than testing with NI MAX. The counter output is routed externally to gate the counter input, and its frequency is set to be equal to counter input rate. Both cards, I can achieve 2 MS/s on NI Max. However, I only got 500 kHz with NI 6602 and 30kHz with NI 6259. Why is that? (If I increase the rate to higher value, the error message "data is overwritten before it can be read out" pops up.) Codes:

s= daq.CreateSession('ni'); s.addCounterInputChannel('Dev1', 'ctr0', 'EdgeCount'); ch= addCounterOutputChannel('Dev1', 'ctr1', 'PulseGeneration'); s.addClockConnection('External', 'Dev1/PFI9' , 'ScanClock'); s.Rate= 500000; ch.Frequency=500000; data=s.startForeground();