I have the same issues as the parent comment (maybe not quite as bad) despite also having a threadripper, 2080ti, 64GB of ram, and my photos living on a 10GbE connected NAS that's gets 1000+ MB/s. Similar issues when I move things into my pcie SSD I use for caching and temp storage.
I work with 8k raw video frequently for my day job and lightroom is by far the slowest app on my system my orders of magnitude.
Not saying that it is the problem here but often IOPS are the bottleneck. Your NAS may read 1GB/s sequentially but if it filled with HDD in RAID it will likely have terrible IOPS. And if you are accessing many pictures all over the platter that may be a problem.
I work with 8k raw video frequently for my day job and lightroom is by far the slowest app on my system my orders of magnitude.