Solution
You can benchmark how long it takes to iterate through a while loop by using the Tick Count (ms) VI.

This takes the current time in milliseconds and subtracts it from the time during the previous iteration, which is saved in the shift register.
You can code an alternate version that uses a Feedback Note instead of Shift Registers. You just need to drop the following code inside your while loop and get the Iteration time (ms) without any modification to your original code.