IngestPreview speed depends on the camera format, available embedded image, storage, and connection. If browsing is slow, try a local copy to distinguish file-rendering work from a slow card reader, network volume, or file provider.
Thumbnails that are expensive to build are cached on disk, under 2 GB, and cleared from Settings, Advanced. Clearing the cache is safe and never touches a photograph.
A photograph with no thumbnail at all is usually a raw format Ingest cannot read yet. See cameras and files it reads.