1) Hide the Histogram panel when running actions or scripts (testing or not). Hiding it drops the diglloydMedium/Huge execution time by about 10%. Even in the middle of an multi-step action, Photoshop updates the histogram display after each step, which can be a time-consuming operation, especially with CacheLevels=3.
2) Use the Bigger Tiles plugin in conjunction with DisableScratchCompress plugin. Failure to use these plugins results in a penalty of up to 50% on Mac OS X in any “decent” configuration (over 100% for Photoshop CS3). Download the Plugins. It’s bizarre that Adobe calls these “legacy” plugins, because they have a massive performance impact.