It's unclear exactly what you have done different to make it run slower; have you always had that subVI there and called it, the only change being that it is set to dialog...or have you done other things too? Wehn you say things are slow - is it just that pop-up VI or does the whole machine slow down?
Do you have a wait function in the dialog VI? If you did not have that dialog before and you have made it with a loop without a wait in it that will explain the slow behaviour. If you can speed things up just by chaning the window apperance though it's more difficult to see a reason. If you could post the sub-VI and the caller as a .llb file or as pictures we could take a closer look.
