From our experience (~15 orders) it is normally faster (or at least not slower), with only a few exceptions. Even in the times where it was about the same, it was nice to be able to get the order for everyone ahead of time, and then just press the "I'm here" button when we get there (or are <5 minutes away).
Our only experiences where it was worse was due to a really long Mobile Order line (Aloha Isle) and due to a Cast Member not doing things properly and taking people out of order (Catalina Eddie's). Otherwise it has been better, especially a few times at Flame Tree, Cosmic Ray's and Columbia Harbour House.
We also have had very good experience with mobile order being faster. It is very convenient to set up the order with family while waiting for a ride or show, then click Im here. I usually clicked I’m here while walking to the restaurant(<5 min away), and had a very short wait for food. Only at flame tree did I have to wait a bit for my food, but at that point in time, the line for non-mobile order was so long it didn’t even fit in the extended que, so I’m sure though my wait was long there, it was still shorter than their wait
We had good luck with it last March. It was still new, so some cast members seemed uncertain and I think that may have made it take slightly longer. Hopefully, that's corrected itself by now. There was an obnoxiously long wait at Backlot Express, but they had our order ready for us when we walked up.