Geez you guys whine a lot. I just cut about 10 4" holes in 1/4" treadplate, thru multiple stacked layers with my mag drill, it wasn't fast but it sure didn't take me all day either.
OP, I'd advise you to heed the advise of the poster above who suggested some chip relief holes, I'd go so far as to suggest 4 placed 90* apart rather than 2x 180* apart. I'd also suggest that if you can do this near enough to shop air or a portable compressor, set up a cool mist or some other mist coolant and maybe an additional chunk of loc-line on a mag base for an air blast to help keep it cool.
Also, buy some GOOD hole saws, not the cheap woodworking crap you get at your local hardware store. I've had best luck with Bahco from a nearby industrial supply shop but Morse is pretty good too.
Edit: If you have a real sturdy mag drill and there's a budget for tooling might also consider getting yourself a single point "grooving" tool attached to a pilot bit in the center, like a piloted trepanning tool, might be faster and clear chips quicker.