Could it be an option to apply a varying amount of Gaussian blur to Preview images?
I've tried the various different resolutions and quality settings and the 1:4 and 25 quality is fine for size/speed, but it's quite blocky. I coud increase quality, but filesize increases accordingly.
Taking an original tile and shrinking it 1:4 and applying some blur, then adjusting the quality settings to get about the same filesize as is currently exported at quality 25 gave more pleasing results. As it takes less data to encode a blurred image the quality setting can be higher for the same filesize, so the JPG artifacts aren't as visible.
A clearer, soft image visible for a second before the high-res loads, is more pleasant than seeing the blocky version before the high-res loads. It's like your eyes are simply adjusting focus, or it's almost a 'free' transition effect.
BTW, a small buglet: changing the Preview settings type (None, Grayscale, Sepia, Color) doesn't export a new version unless you also change some other value, such as quality.