Do you have the hard drives set to turn off after an interval.  If you have then, after that period the thumbnails will display until to change folders at that point the library will wait for the hard drive to spin up, around 10 - 20 seconds depending on the hard drive (assuming it is not an SSD) before it reads the content of the folder..

 

 

I use Poser 13 on Windows 11 - For Scene set up I use a Geekcom A5 -  Ryzen 9 5900HX, with 64 gig ram and 3 TB  storage, mini PC with final rendering done on normal sized desktop using an AMD Ryzen Threadipper 1950X CPU, Corsair Hydro H100i CPU cooler, 3XS EVGA GTX 1080i SC with 11g Ram, 4 X 16gig Corsair DDR4 Ram and a Corsair RM 100 PSU .   The desktop is in a remote location with rendering done via Queue Manager which gives me a clearer desktop and quieter computer room.