The execution rate of your code is directly dependently on how long it takes the code inside the loop to run. Your first iteration is going to be zero because the tick count before the loop and the one inside the loop will run at almost the same time. After that, the time it takes for then tick count will depend on how long it takes for LVCanlvdt.vi to execute (it is missing so I can't see what it does) and the write to spreadsheet file.
Writing to a file can be notoriously slow and unpredictable on how long it takes. It is dependent on the Windows OS. If something else like a virus scan happens, it may put a low priority on file writes. If it needs to find another block on the hard drive to write data to, it can take longer. Plus you are constantly opening and closing the file which takes time.
