Hello Toro,
That is a great question. Depending on what controller and modules you have, the execution times of the modules vary from one to another. There is an excellent tutorial that explains this really well, below you will find a link to it:
http://zone.ni.com/devzone/conceptd.nsf/webmain/a14d3c3314a0e24a86256dbe00664943?OpenDocument
If you don't have a real time module, then by switching to one, you are going to see a significant change in the accuracy of the times displayed. Please let me know if you have any further questions.
LA