Tjyylli

In my experience it takes less time to reheated a cooked item than it is to cook it. This is true for every single different "type" of cooked item I can think of. (Meat, soup, pasta, beans, etc etc).

It's quite common for my to use the microwave to reheat things, and that might lead me to be biased in thinking that it's faster because the microwave itself is often the fastest way top reheat something, but this observation isn't just for microwaving. It doesn't even seem to matter on the method of reheating, as I can reheat something faster if I use the same method of as I did to cook it (e.g. by frying).

Note that I always check the temperature of something I've reheated via a food-probe, so I'm also not making a mistaking of cooking something to 70C and then reheating to 45C etc.

So:

1. Is it always faster to reheat something than it was to cook it, or are their exceptions?


2. 
   why is food faster to reheat? What's the food-science behind it?

"Cooking" is often a chemical process. Denaturing proteins, gelatinization, causing chemical reactions like browning, or even causing state changes like evaporation.

In many cases for these reactions to happen, we need to overheat the food. (Cook it and let it rest to cool off back down to undo some of the changes that were made and/or bring it back down to a reasonable temperature to eat). This is true when grilling meats, frying, baking bread, and lots of other types of cooking.

Other times, we need to bring something to temperature and hold it there for some period of time. This holds for extracting collagen, starch gelatinization (eg, cooking pasta, potatoes, etc.) but also just waiting for flavors to transfer in soups and similar dishes.

With warming, you're just adding enough heat to it to move it a few degrees, but you're not typically trying to change the state of the food, so less total energy is needed.

Now, it is always faster to reheat vs. cook things? For the most part it's true, but I suspect that there would be an edge case out there. Something that's cooked from room temperature, but then stored chilled and the chilling causes issues (like retrogradation in starches, maybe?) that make them more resist than reheating.

This is because when you're cooking some foods you're not just heating it up. A lot of foods are boiled, not because they need to be heated up, but because they need to absorb water. We just boil the water because that makes the hydration go a lot faster (the high temperature is also needed to break down some of the starches, for more info, see here).

With soup it should take about the same time, if you don't care about dissolving/softening the vegetables into the soup. That also takes time, with vegetables the chemical reaction involved is mainly breaking down the pectin that holds the cells of the vegetable together.

With meat, dissolving/denaturing the collagen (stuff that holds everything together) into gelatin also takes time. Also you want a different temperature for reheating than frying because with meat you want a nice crispy brown outside (Maillard reactions), and for that you need far higher temperatures than the inside of your meat.