The more restrictive your dietary preferences (kosher, paleo, vegan, low-carb, pick your food fetish) the more benefit there is to learning how to cook and spending time cooking. Almost any restaurant, and almost exclusively any restaurant that prepares food quickly, uses massive amounts of corn starch, vegetable oils, sugar, flour, grain fed meat, milk, farmed fish, Crisco, and other ingredients I no longer want to consume. If you're not averse to eating fast food, then you may well save time by eating at Wendy's, Popeyes, and Au Bon Pain.

For me, although I live in the largest city in the United States, I know of zero public restaurants that are truly appropriate for someone following a low-carb diet. It's mostly a question of how many substitutions can I get away with, and how many of the menu items are plausible. If I were really serious about grass-fed meat there'd be almost nowhere I could eat anything.

Once you've decided to cook, it does pay off to learn a few dishes that you can cook well and quickly that you're happy with. If you can cook them in mass quantities and store them for later, so much the better. For breakfast, I can whip up eggs or omelets (several styles) far faster than I can get the same from the nearest diner, cleanup time included. I'm still working on figuring out lunch and dinner.

Cooking also benefits from economies of scale. Cooking for 2, or 4, or 17 is cheaper and faster per person than cooking for 1. Eating out, by contrast scales linearly in cost.