Many major roads in Africa and other regions in the developing world are unpaved (up to primaries, but certainly most secondaries). The surface makes a huge difference on driving speeds and considerable amount of the highways in osm carry a surface tag. If I figure correctly the surface is not yet taken into account by OSRM.

Therefore I would suggest adding a penalty (e.g. 0.6) for unpaved highways. Since there are multiple values for unpaved in use, I would suggest treating the following values as unpaved: unpaved, gravel, ground, dirt, sand, compacted, pebblestone.