Thanks for your replies.

After a little brainstorming I decided to try building the cache in '/dev/shm' and then move it to harddisk once it was complete. This had the ever so pleasing affect of reducing the build time from 15minutes to 15 seconds! It also reduced the size of the built out cache for some reason. Took it from ~70MB to ~38MB?! I Can't even explain that but I'll take it.

Anyway, I thought I would post this here for posterity in case anyone else needs this optimization in the future or if anyone would like to further discuss this method.