Best resolution that compromises Speed and Graphic quality.

Say you are making a HD game to match the 16 by 9 ratio of today's phone/tablet/WiU/Whatever.

What would be the resolution that you would use that compromises smaller resolution to speed that will be playable on most retina devices today ?

1920 by 1080 [which is insane at this point for HTML 5 games]

1280 by 720 ?

Or even smaller ?

How small will you go before you think to yourself "why don't we all go back to the 1980's if you want to make it so bad".

What is the optimum resolution for most of your 16 by 9 games for retina fellow game makers ?

I understand some might say "it depends on the complexity of the game with its bla bla bla", please don't.

All things being equal [which they are not I know], what is the usual resolution you use to make 16 by 9 games now ?

I was doing some research and for graphical fidelity 1280x720 seemed rather good. Not sure what happens when you get A LOT of objects on screen, but that should be reasonably future-proof for a while, given how the mega-resolution tablets and phones have such high DPI that the tiny pixels in your art wouldn't be noticeable anyway.

Yeah I am thinking of 1280 by 720...yes, having to compromise future prove [2 years hopefully ?] with playability.