How to cut copper pipes quickly (and cleanly)
A compact miter saw with an appropriate blade would make quick work of things. You'd still have to clean up the cuts to eliminate burrs. 

You could also use a circular saw, but you'd want to take care to keep your cuts square. 

Most plumbers don't want to lug that stuff around the jobsite, though.  (Even compact cordless circular saws are not something that fits in a tool belt). Once you get skilled with a tubing cutter it's a matter of about 10 seconds, even with 3/4" or larger pipe. You aren't going to reduce that enough with power tools to net much time savings.