I did reach out to Jim and he had some useful information to share with me. He did not recall clearing the cache based on his past experiences. But, Victoria (aka "the LR Queen" also ran tests where she cleared the cache and had similar results. Jim also ran one of the series again for me last night/this morning while clearing the cache and here are his results:
1. Files on Internal SSD: 2.20 secs per image (2.28 secs on the previous test).
2. Files on External SSD: 2.25 secs per image (2.28 secs on the previous test).
3. Files on External USB3 hard drive: 2.23 secs per image (2.24 secs on the previous test).
There was minor improvement, but not enough to really make a significant difference. To sum up the findings in Jim's words: "the file read speed is just not a factor in the overall preview-building process. Frankly, the drive could be read at Warp speed and it still wouldn't make much, if any, difference. Preview building is almost all CPU driven, so using the GPU instead of the CPU might help, as we've just seen with AI Denoise (and with export performance), but until then I can't see how the user can improve the preview building performance."
Hope this helps,
--Ken