It works best with 1440x900 resolution. So if you like 1680x1050, your GPU will have to do extra work.
The display's native resolution is 2880x1800. Why would you set it to half the native resolution? That defeats the whole point of having all those pixels! You might as well have a 1440x900 (non-retina) display.
Setting the display output to 1680x1050 is totally whacked - that is displaying each pixel in 1.7 native pixels. Mind. Boggles.
When it comes to Apple's Retina displays there's a bit more to it than what you're used to:
- when OP says 1440x900 he means the pixel doubled version. All GUI lengths / fonts have the size of their 1440x900 versions, but there are twice as many pixels, enabling high resolution graphics and fonts. It's basically a crotch because Apple never got around to make their OS resolution independent, however from a programmer's standpoint it's actually not that bad, you just have to specify images with @2x postfix in the filename.
- higher resolutions are not scaled in the same way you're used to. They're actually rendered internally at double the resolution after which they're overlayed with the screen's native grid for some interpolation-like process. The result is vastly better than what you know from usual screen interpolation, it's hardly noticeable actually.
Relative resolutions. "By default, the new MacBook Pro ships in a pixel-doubled configuration, meaning you actually get an equivalent to a 1440-by-900-pixel resolution. This offers no actual increase in desktop space over that of the current standard non-retina MacBook Pro, but the increased pixel density means that items onscreen can appear much sharper than they did before." [1]
The point of having the pixels is so that stuff looks sharper. 1680x1050 is a lot more screen real estate = more job for the windowing system to display everything nicely. Same thing for running it at full 2880x1800 resolution.
Set OS X to use half-res and check your images again. They should be rendered at 2x (meaning they actually use all the pixels not a quarter of them) so they shouldn't look fuzzy at all.
The display's native resolution is 2880x1800. Why would you set it to half the native resolution? That defeats the whole point of having all those pixels! You might as well have a 1440x900 (non-retina) display.
Setting the display output to 1680x1050 is totally whacked - that is displaying each pixel in 1.7 native pixels. Mind. Boggles.