I was only seeing the low quality image but I'm using imagick in testing, so maybe GD is different (1.1.9).
The issue stems from those tmp files we use when in preview, they were originally intended for use in the upload area, so they are just 100px thumbnails. When they get ila'ed in preview above 100px they look horrible!
A quick fix to try is in Attachment.controller.php
Code: (find) [Select]
if ($resize && resizeImageFile($filename, $filename . '_thumb', 100, 100, $format))
Code: (replace) [Select]
if ($resize && resizeImageFile($filename, $filename . '_thumb', 250, 250, $format, false, false))
That will provide a larger thumbnail image for use while still remaining a very small file so things do not slow down. Give that a test and let me know, I'm testing it as well.