There must be something in your loop that is serializing access. It could be as simple as a subVI that is not reentrant. The execution of one loop must wait for the subVI in the other loop to complete. If your delay is inside a VI "wrapper" that is not reentrant, you'll see that speed cut in half with two loops. When I create a VI with two parallel while loops with "Wait (ms)" functions in them, they run at the same speed.
Daniel L. Press
NI Certified LabVIEW Developer
PrimeTest Corporation
www.primetest.com