When 'maximizing' a Minecraft window, my framerate drops to a consistent 15 fps when I'm otherwise able to run at around 60fps in most other situations.

Steps to Reproduce:
1. Start Minecraft
– Edit: Set Render Distance to 'Normal'. –
2. Hit f3 to bring up the stats display
3. measure fps.
4. 'Stretch' the window so that it's almost maximized, but not quite. Measure fps.
5. 'Maximize' the window via the windows maximize button or double-clicking the title bar. Measure fps. Note the massive drop.

In the screenshots below, I've posted examples of what this looks like. In the first, 2013-09-06_15.44.56.png, it's the little bitty window that Minecraft always starts with. 58 fps. This is on the new 'amplified' world setting, just after I've created a new world, so it's working pretty hard.

In the next, 2013-09-06_15.46.01.png, I've stretched it out so that it's not quite maximized. 60fps

In the third, 2013-09-06_15.46.19.png, I've 'stretched' the display across two monitors. 59 fps.

In the last, 2013-09-06_15.46.34.png, I've moved the display back onto the main monitor (powered by the GT430) and simply maximized it so it's slightly larger than the 2nd. Drop to 15fps.

So something about 'maximizing' the window is arbitrarily limiting the FPS there, especially if I can drag it across multiple displays for a larger total pixel count.