Latest PL. Finding thumbnails often remain black with a circle in the centre and only show an image when I click on them. The images are on my main drive on a late 2017 iMac. Seems to be happening in Projects only and once I have clicked through the images they refresh fine, even after closing and restarting PL.
Yes, as thumbnails are calculated based on your default preset, and not just using the jpeg preview inside the raw, it takes time to display all, especially if you have a great number of images in your folder.
If you wait you can see those which are on the screen becoming visible, time depending also if you have modifications while working in your current image.
It also depends of the size and definition of your screen, and the size of your filmstrip.
For me priority goes to the current image modification, and filmstrip when possible with lower priority.
Thanks Pathal, I have refreshed all the thumbnails and deleted some where the image no longer exists and it seems better now. Basically, I tidied up the project file.