It helps to have a flight plan to reduce the delays. If you are trying to access Class C airspace and don't have a flight plan, ATC pretty much have to put one in for you, which means they need to ask you all the details which you could have already done in NAIPS. Expect delays while this happens, and if the controller has a bit to do already submitting details comes last in the queue.
For Class D, a paper strip has to be written out, this can take some time if the controller is busy with something else. If you are doing any sort of long distance or Nav flight, submit a flight plan in NAIPS, it only takes a couple of minutes, it's free and it may just save your life one day.