That would probably explain the slow-down/freezing when inserting images that have already been added to the epub archive incorrectly (by inadvisably copying them to Sigil's temp space).
If you recall, we had another user who was doing the same thing when we discovered that he had a third-party shell extension of some kind that was causing issues with the thumbnail generation feature of Sigil's Insert Existing Image tool.
Never understood why anyone would think messing around in a program's temporary scratch-space (while said program was running) was a reasonable idea.
