Potentially the long delay is because the "fetch time" you are measuring is a combination of the acqisition and fetch time. With how you have your code written, there is no guarentee that the aquisition is complete before you call niScope fetch. In this case, your fetch is going to wait until the acquisition is complete, and then return the fetched data. To ensure that your fetch benchmark is really just the fetch, you should use the niScope Acquisition Status.vi after you call niScope Initiate Acquisition.vi, and wait until it state that the acquisition is complete before fetching the data.
Another test you could try is increase the frequency of your trigger pulse to the devices.
Good luck!
Systems Engineer
SISU
