Yes this was a more sparse area and having been editing more detailed areas over the last few hours I agree that the tiles don't seem to be re-rendered so quickly in the more heavily mapped areas.

I wonder if this is something to do with the way the tile system is set up to render. The way it works, if I understand it correctly, is when a tile is viewed it is checked for freshness. If it's not fresh then it's queued for re-rendering. Tiles in urban areas are more likely to be viewed often and OSM data is generally more detailed in urban areas so that could be the cause of that affect.

Regardless it's a tremendous achievement for OSM to render tiles so very quickly at all and I also hope that it continues. :)