Comment by brigade

Thumbnailing happens in a background process and shouldn’t block the main thread; rather hangs are usually because fetching xattr over SMB is slow and Finder will block until it’s enumerated the metadata of every file in a folder.

Preview actually does use progressive rendering for large enough images, even from SSDs. You can tell because opening an image will be blurry for several seconds, even when the full size image could have been decoded in a quarter second or less.