That's exactly my situation. Going to 720p requires half the fill rate of 1080p. Turn off some postprocessing and antialiasing and you might get double the performance again.

So if you're at 1/4 peak performance, you're still running a smooth game albeit not quite at full detail.

If you're at 1/16 peak performance... things aren't looking too good by the time you get your game smooth.

And that was exactly my point :) In case it wasn't clear from the context, I was suggesting getting a notebook with a discrete GPU instead of integrated GPU (that's what iGPU/dGPU stand for, not a desktop GPU).

If you are game developer / graphics programmer / game player / designer / photo or video editor / VR enthusiast / anybody who cares about graphics, I would advise to get at least GTX 960M (if you have budget or size constrains, this should get you roughly to PS4/Xbox One levels, so relatively safe for this gen), but preferably GTX 970M (best price/power and you can still get slim notebook).

GTX 970M will get you about average GPU performance overall, including desktops (these days I get ~50-percentile rank in 3dmark firestrike with my GTX 970M notebook, so average GPU in September 2015 is at least as fast or faster, at least for that cohort of people).