I see these things called "roughing mills" that look sort of like a tap. I
assume they're end mills that are optimized for removing a lot of material
fast, rather than getting a good finish. Am I right? And if so, what are
the particulars - like, how much of a cut can they take, and how long before
they get dull? How much better are they than a regular end mill?

The thing that prompts the question is that I have some 5"x5"x3/4" steel
(10L12, I think) blocks that I want to take a 4"x4" notch out of the middle
of, to make a "C" shape. I was going to use a bandsaw, but I don't have
access to it any longer; all I have is a small mill/drill and a (manual)
hacksaw. I could chain-drill and use the hacksaw, and then finish with a
mill; but I was wondering how else the problem might be approached, and got
to thinking about what it would take to just take it all out with a mill.

Thanks!