You'll hear a lot about speeds and feeds, but a term that I only started hearing/learning about recently is MRR (material removal rate). Basically, how fast can you chew up metal, given YOUR machine and setup. For instance, say you swapped the 1Hp motor for a 3Hp? Your MRR goes up, unless the whole machine starts flexing. That is a real thing, and is why I now have a BP clone instead of an RF-45 clone.
So, I learned about MRR. I have a 1-1/2" endmill that I got on auction for $5, then had to spend $40 on a holder for. I would use it to cut a T-nut, but I'd still take only a .25" wide/ .25" deep cut, 'cause that is what my setup with that cutter will handle. If I swapped out for a sharp carbide tool, I might be able to double that. The point here is that the size of the endmill doesn't necessarily matter, unless your doing a slot cut.
How do you learn the MRR of your machine? Broken endmills and a notebook. I purposely set out to get an idea of the limits of the machine, and had a few regrinds that I was willing to sacrifice on the alter of education. Keep increase the cut till things get wonky, usually resulting in a broken endmill. Listen to the machine and the sound of the cut. It will tell you what you can get away with once you understand its language. And, write down your results.