If you have a boarding pass in hand, as long as you can race through security and make it to the gate before they close the door, you'll probably get on. But if you get to the airport too late without having checked in, you might miss the check in window. In that case, you'll have to argue with someone at the check in desk because kiosk check in will have closed. That alone can take a lot of time with reduced staffing at airports these days.