For generic performance and trouble-shooting advice I’d like to recommend reading these:
http://helpx.adobe.com/photoshop/kb/optimize-performance-photoshop-cs4-cs5.html
As for the specifics:
The image size is 4.31M. If I go to double the size and change nothing else, the image size jumps up to a whopping 17.2M.
If you double the dimensions of an image the area is increased fourfold – so what is the problem there?
The image dimensions are 500px x 135px, 300DPI, 8bit, RGB color. This is at 100% zoom. Those are the settings I normally use, but for some reason it’s showing the file as about half of the size it should be. (Tumblr post for comparison—they are 500px wide.)
The introduction or Retina (and other high resolution) displays and browsers actually up-scaling images to accommodate those screen resolutions seems to have confused some users about the concept of pixels.
Could you please post a full size screenshot on this Forum of the image at View > 100% in Photoshop and on tumblr?