Cullibrate makes thumbnails from the preview images your camera embeds in each RAW, using the smallest one that is large enough. Most cameras embed them; a few don't.
If thumbnails are blank
Your camera may be writing RAWs without an embedded JPEG. Enable "Save JPEG" (or equivalent) on your camera — this is usually a menu setting under Image Quality.
If thumbnails are slow
- You may be on a slow external drive. Move the file collection to an internal SSD to test.
- Cullibrate keeps thumbnails in its disk cache after the first pass, so opening the file collection again is much faster. For very large collections, raise the Thumbnails & previews limit under Disk cache in Display & Performance preferences.