Cullibrate reads the highest-resolution embedded JPEG from each RAW to render thumbnails. Most cameras include one; 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 shoot to internal SSD to test.
- Cullibrate builds a cache on first pass. The second time you open the shoot is dramatically faster.