Reboot does not help
The weird thing is that even thumbnails which have been shown before in the same dialog session are still delayed when you hover over them the second time.
If it is cached in the default temp folder, it is not diskaccess time either since that is an SSD
I think the biggest performance issue is that when you hover over the list, you get intermediate busy delays wich take 5-15 seconds and that happens a few times if you have a long list like Misc
If you hover over each of them one by one then the delays will be gone during the poser session (whether you leave the plugin or not). Once you restart Poser the delays are back again
If I speculate it seems like it is reading a bunch of thumbnails in blocks and starts reading the next bunch when the mouse ecounters a non-cached one.
Configuration: Windows 10 64bit Creators Edition, I7 6850K, 128GB, 2 GeForce 1080ti, P11 Pro SR6