Calling a subVI from a VI running on an RT target adds a small amount of overhead to the overall application. Although the overhead is small, calling a subVI multiple times in a loop can add a significant amount of overhead. You can embed the loop in the subVI to reduce the overhead.

You also can convert subVIs into subroutines by changing the VI priority. The LabVIEW execution system minimizes the overhead to call subroutines. Subroutines are short, frequently executed tasks that generally do not require user interaction. Subroutines cannot display front panel data and do not multitask with other VIs.