Paul,
I experimented with importing a large group of items, and looking to see whether preview generation shows up on any of the PR background process tools. It did not: Progress panel and Progress Dashboard are handy, and they will show activity that is completed. But nothing for preview generation. And nothing shows, as expected, in lower right of screen where pek file generation shows.
Also, the number of mp4 files I had available per folder was not high enough to slow it down very much. But I could see that previews were not there is the beginning, and taking some number of seconds to be generated.
I would look at performance indicators - CPU, GPU, drives just to see where the activity is.
Stan
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