PNGs from https://maps.wikimedia.org/osm-intl are sent as 32-bit RGBA. This is not ideal, as the map tiles contain no transparency and don't generally need 32-bit colour.
The least that should be done is setting the Mapnik format to "png32:t=0" which is 3 8-bit channels, no alpha.
We can probably farther and use paletted png256. Because each tile is paletted individually, the quality is maintained, and tile size is 50% of full-colour PNGs. tile.osm.org uses png256 and has a style which makes much more use of colour.