I've done all of mine on a milling machine. I actually have three I need to process currently and another three on order (one AR10, one AR9, and one AR-45). I've got another AR9 and two AR15 80% lowers coming (soon I hope). Takes about an hour to process the lower on the mill. Including drilling and reaming the side holes. IIRC, it takes at least a couple/few hours to do with either the drill press or trim router. I've not used any cooling/lube in the past on the mill (I've heard of people using WD40 with the trim routers). IF I need to, I have a mist cooling system already in place that will be easy to employ.
I would make damned sure your drill press spindle is trammed to the bed of whatever vise you're going to use. Also use under sized drill bits and ream the side holes if you want to get them the correct size. Drill bits are NOT 100% round and can deviate from the listed size by enough to make it a PITA. I trammed the spindle to table and vise bed on my setup last night and made sure the vise was square to the spindle earlier today.
BTW, a few years back someone walked into a gun store I was in looking to get help with the 80% lower he had been spending several hours on. Hammer wouldn't fall properly. He got tired of using a file and Dremel to attempt to get it correct.
Yes I know people can do it with the drill press or trim router options. Just seems more complicated (to me) than using a milling machine. I also realize that not everyone can have a milling machine in their garage/home shop. Luckily I do have one.