See T191805#9230056
When testing uploading an 8.5 gb file with Filesystem backend, a surprising amount of time during file operations were spent doing sha1 calculations. At first glance, it appears often these calculations aren't even used. (They were also already known to MW as we have them in the DB).
This was taking about 2-3 minutes to calcuate. Was about 1/3 of a Copy operation (aka moving an image page), and 100% of a MoveOp (e.g. deleting an image).
At the very least, we should only generate them if we actually intend to use it somewhere