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[Stadi_Thompson]

Seem to have mixed results on block size. On Nvidia card the higher the block size the better the FPS. Our programmers both have ATI cards, seem to be opposite results. I get 20-40 more FPS with 128 or 256 block size vs 64. ATI guys are experiencing significant loss. Anyone know much about Merge/Regrid option in the Terrain Panel?

[HE-CHRISTOPHER]

The results will vary based on the environment and topology of an area, not to mention the capabilities vs load on a particular graphics card. 

Smaller terrain chunks allow for (under some circumstances) more optimal culling to be performed with a trade off in additional draw calls and higher overhead.  Larger heightmaps require fewer draw calls but are more likely to not be culled since a portion of the heightmap is often visible increasing the (a different type of) load on the renderer.

As for what is more optimal for a given area, the only way to know is experiment with the tradeoffs to see what is best.

[Stadi_Thompson]

Thanks for shedding some light on that makes perfect since now.

[JoshHalls]

It was kind of interesting as the Nvidia card seemed to handle larger prim counts, but draw calls were more impactful vs the ATI card which could handle more draw calls vs prim counts (before the frustum fix this was very evident).  This is all somewhat relative as it was > 60 FPS, but there probably is a sweet spot as you mentioned.