Firefly looks outside of the current bucket to check for stuff which would influence the current bucket;  an example would be displacement materials.  So,  instead of just looking at the 8x8 bucket, Firefly actually checks a border of a few pixels around the bucket for these effects.  That means some duplication of effort.

That border is less significant if you're using a 32x32 pixel bucket.  Less duplication of effort -and less frequent fetching-  allows the faster render - if you have the RAM to support it.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5