If your tickets are linked and your credit card is stored, you can do it in seconds while you walk (assuming no IT issues). You’ll open the app, click on the “Get Fastpass” icon on the front page, it will show you the linked tickets and prompt you to buy Maxpass, which takes one click, entering your security code, and another click. So it’s like four screen touches and a security code to buy it.
ETA: If you’re hustling to RSR, don’t try to do both; grab it once you’re in line. You have to move purposefully for that one.