The four things that belong above the fold on a phone
What kind of restaurant you are, where you are, whether you're open, and one obvious action — order, reserve, or call. If someone has to scroll to find any of those, the page is costing you customers who were ready.
Test it yourself on your own phone, on cellular rather than the restaurant's wifi. The gap between how a site feels on office broadband and how it feels in a car park is where most of the loss happens.
The PDF menu is the most expensive habit in the industry
A PDF menu opens slowly, pinch-zooms badly, can't be read by a screen reader, and is invisible to search. Nobody can find your dishes and half the people who try give up.
An HTML menu fixes all of that at once and is one of the few changes that improves both conversion and search visibility in the same move.
Tap-to-call, not a phone number
A phone number written as text is a number someone has to copy. Wrapped in a tel: link, it's one tap. It sounds trivial and it's routinely the single highest-impact fix on a restaurant site.
One ordering call-to-action, in the same place, every page
Restaurants often have three competing buttons — order, reserve, join our list — with equal weight, and a fourth ordering link somewhere else that points to a different provider. Pick the action worth the most, give it visual priority, and keep it consistent.
Hours that are actually right
Hours on the site, on Google, and on the door should match, including holidays. Conflicting hours generate more bad reviews than bad food does on a normal week.
If you can't see where people drop out, you're guessing
Conversion tracking on the ordering click, the reservation click and the phone tap. Without it, every conversation about the website is opinion. With it, the problem is usually obvious within a week.