I would have expected that result.  When you use such high values for minimum displacement bounds, you are directing Firefly to look far beyond the edges of each bucket for displaced geometry.  The min displc bnd must be high enough to cover the furthest displaced detail, but any setting beyond that is wasting render time.

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  Instead of looking only at an 8x8 pixel bucket (64 pixels total), your settings may (for example) instruct Firefly to account for perhaps a four-pixel boundary outside of the bucket.  That means 256 pixels total.  It is unexpected to you that this would take longer to render?

     While it may be feasible to work Arial or similarly simple fonts into the mesh, it would not be so with Hurricane or other flowery cursive fonts.  In general, florescences and such organic shapes are very expensive in terms of poly count.

     I don't shy away from using more polys -modeling detail into the mesh- when the cost/benefit ratio is favorable.  Firefly will not not slow appreciably from more polys until you begin to approach the memory limits, in which case it will bog down terribly, while a simpler mesh with displacement detail will continue to zip right along.  In such a comparison, it is likely that a 64bit workstation would render the mesh-detailed model quicker than the displaced model.  However, the common 32bit computer may struggle with the high-poly model, and so render the displacement model more quickly. 

     In practice, I model details into the mesh if it is necessary to establish shape, or if it can be done at modest poly count.  But for something such as an armor breastplate with a curling flowering vine design, I would use displacement for the design.  I try to consider that many end users will have only modest machines, and build models that most everyone can use.

     By the way, since the advent of Firefly IDL (P8 and later), I now model trees with mesh leaves rather than transmapped billboards for the foliage.  I tested both versions of my trees, and when rendering with IDL engaged, the multiple overlapping layers of transparency slows IDL renders down terribly.  I  have found that it is "cheaper" -in terms of system rendering resources- to use mesh-modeled leaves than to use the much lower-poly transmapped foliage.

     Your experiment is interesting (I do a lot of that sort of thing myself) and a worthwhile investigation.  Try using a min displc bnd of 0.25 or whatever is just sufficient to cover pixels displaced beyond the bucket.  You might also try another test comparing 100 mesh-detailed cubes versus a render with 100 displacement-detailed cubes.

Poser 12, in feet.  

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