Three things to surely slow down Firefly renders:
- Shading Rate <= 1. I personally don't believe smaller shading rates (0.2 for example) are necessary, with the possible exception of transmapped hair. Also Shading Rate can be set on a per-object basis.
In fact, a very large and influencial animation studio usually uses SR of 1 for foreground objects, and SR of 5 or more for midground / background objects. You can increase the SR to 10 or higher for motion blurred or objects outside of the Depth Of Field.
- Incorrect bucket size. Try powers of 2 numbers in the bucket size. Shading rates smaller than 1 will dramatically increase the bucket size and waste render cycles. Start with bucket size of 16, Shading Rate of 1 (which should be pretty speedy) and up the bucket size to 32, Shading Rate of 1 if you have gobs of RAM.
(Bucket Size ^ 2 / Shading Rate) will determine how large your rendered tile will be, how much RAM is used per tile and how fast the overall render will be.
- Large Pixel Sample value. Keep it small for drafts (3x3) and increase it for final renders (maybe 9x9) and only increase it if you still get anit-alias artifacts, especially if objects are affected by Motion Blur or Depth Of Field.
