The most efficient use of the favourites list on your phone is to store the numbers you call the most often.
That's not how I use mine. I actually use it for my favourites.
When I fly I generally park at Jetport Parking (thanks Rohan Dredge for that tip many moons ago). When I land I have to call them to let them know I'm back. So that's a number I call pretty regularly, but its not on my favourites.
My mate Jez, on the other hand, is. And I'd be lucky to call him every three months. But he's been one of best friends since uni, we've shared countless adventures, he'd have my back no matter what. He’s one of my favourite people on the planet, and he makes it to my favourites on my phone.
Lisa O’Neill (who is also on my favourites tab) has a fabulous exercise in her workshop called “top 20”. She has a slide of her top 20 people, and she talks about people who contribute rather than contaminate. She then gets people to create their own top 20.
I think the little boost I get when I see the little photos of my favourite people, the little spark of joy, is worth taking a productivity hit for.