It would be good to have the option to bypass the calculation of a file hash code if the size has not changed and the timestamp has not changed (just the timestamp should be ok). This has huge performance potential... avoiding doing work at all.

Possibly there could be some trigger that the file timestamp must be older than an hour or something... to avoid user interaction problems.

NIO allows a fast mechanism for collecting bulk metadata, it should be possible to traverse a directory and get all of this information very quickly before deciding what needs to be rehashed.