Does steak from a supermarket cook quicker than steak from a butchers?

Going to a butcher is a very good idea, and doing some of your own butchering on larger cuts is a great way to save money and get really good quality meat. It's not a question of butcher versus supermarket though, the factors that make a difference in cooking time are:

• Cut: The word steak is a generic term for a small cut of meat cut across the muscle fibers, these cuts come from many different parts of the animal and have very different properties. Some cuts are denser because they do more work on the animal, these denser cuts like rump/round take longer to cook. Skirt steak does not cook at the same rate as filet, you can't compare them directly
• Aging: Most of the time steaks are aged, this can be using a wet or dry method. Dry aging allows beef to loose moisture, concentrating the flavor and making it denser. Wet aging is cheaper and faster, but does not allow for this moisture loss. Dry aged steaks will be denser than wet aged steaks of the same cut, and therefore take longer to cook. There are plenty of butchers and supermarkets that sell both wet and dry aged beef, butchers are much more likely to be selling dry aged beef because you buy from a butcher when you want something better. Note that you can dry out wet aged steaks a bit by leaving them unwrapped in your refrigerator for a day, but it's never going to be the same quality

The key message is not to cook to time, but to temperature as all steaks are different. A good instant-read thermometer is well worth the few bucks to get the right result.