New PC build for LRC PS

One very important factor that I don't see here is the interface speed and size of the main SSD drive. This is a major determinant in how fast Lightroom feels. I would not go with anything less than 1 TB NVMe type although 500 GB probably will work OK if you have lots of other storage and are obsessive about keeping the main drive lean.

My question is, how much of a difference would it have made if built with the 12700K processor instead? 

By @thisisperry

In the benchmark posted by F. McLion, many of the lines indicate “slightly” faster performance, from 5 to 10 percent. In the context of Lightroom Classic and Photoshop, the advantage could be bigger or smaller depending on what mix of CPU, GPU, storage, RAM, and AI/machine learning hardware affects performance the most for a specific feature.

For things like preview building, which is currently mostly dependent on CPU, then maybe previews finish a little sooner. For the Develop module and exporting, the difference might not be as much because some aspects of those tasks are GPU accelerated, which reduces CPU usage. For machine learning/AI features, the GPU and any specialized machine learning hardware in the CPU or GPU might make more difference than the brute force power of the CPU alone.

Look here:

https://cpu.userbenchmark.com/Compare/Intel-Core-i7-12700K-vs-Intel-Core-i7-12700F/4119vs4124

The K version uses more power and has a faster clock rate ... leading to more power.