- you are ordering food and drinks, speed is not essentials, if you're in a hurry you don't sit down in a diner/restaurant
- you assume that everything on the menu is perfectly clear, but what exactly is that thing with the mysterious name? (for example peri peri fries means nothing to me) you can ask to a person, not to a PDF
- you really want X but you have food allergies or some other dietary restriction, again you can ask that to staff, not to a web site
- most importantly, you're assuming that waiting is generally considered an inefficiency, that should be addressed or fixed and that should be the goal of every place serving food and beverages, while it usually is the moment were people sit and relax and have a little chat, it is called lunch break for a reason, isn't it? It's the generalization of XKCD #303.
p.s. in my experience in places that use QR code menus orders are not served faster, actually the opposite is often true.