Solution:

Since the digital input of PCI-1750 are isolated digital input. The response time is slower than the non-isolated one. As you can see in the specification of PCI-1750, the maximum throughput of digital is 10KHz. That is to say the PCI-1750 needs 100 micro second to recognize to input signal. That is why sometime it needs 30 micro second or more to stand a edge signal. That is a hardware limitation, if you need faster digital I/O card, maybe you can consider the PCI-1751.