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Cache Tile Size Is Critical
Photoshop CS5 ships with its Cache Tile Size default set to 128K. That is not a Good Thing for most users.
Best performance is with 1028K cache tile size. There might be some workloads for which 128K tiles are faster, but certainly not for the photography I do.
Testing cache tile size performance
The graph below shows the results on both the 6-core and 12-core Mac Pro, using the four Cache Tile Sizes. All results after a warmup, so these represent best-case performance on the diglloydMedium benchmark.
Behavior is consistent between the 6-core and 12-core, but relatively worse for the 12-core Mac Pro. Note how much more poorly the 12-core performs than the 6-core, see the details.