The difference in computing required between just displaying a list of the files in there vs interrogating each file, generating a thumbnail on the fly and possibly sorting based on some metadata
Perhaps it's just me but I would happily trade in all that gimmickery for a usable file manager on OSX. I don't care for thumbnails.
I think I have tried every finder-addon and replacement under the sun. Nothing comes close to the frictionless and fast experience that (dare I say it) Windows98 Explorer gave me 14 years ago.