Normally you rough drill something like that with are really large drill bit in the tailstock before using the boring bar to bring to final dimension. Once you had a pilot hole stepping up to a 2" or 2.5" drill bit would go pretty fast. I have also use bi-metal holesaws to cut holes in plate, actually goes pretty fast. Have not used one in a lathe... but then I normally don't bore that size hole in plate in a lathe either I am curious, what lathe are you using?
If you mean making them one-piece out of round stock that would be pretty labor intensive to, probably one of the reasons they use cast steel to mass produce parts like these. Get a rough piece, do a quick bit of machining and it's done.