While debugging OSM Scout Server interaction with modRana, I found out that the extra border added to each tile for consistent labeling was scaled in Mapnik by the scale factor. This is in addition to the scaling that I did in the server code (great minds think alike). This slowed down tile generation significantly and was more visible in modRana's case which is asking for smaller tiles.

Next release of the server will have the fix for double scaling that leads to the faster rendering of the tiles.