Renaming (with 'mv') very large files is SLOW

[cygwin@kosowsky.org]

I tried renaming some very large files (20-40 GB) using:
   mv <oldname> <newname>
without changing the directory of course.

The process took about 10-20 minutes with Task Manager showing disk
activity of 100+ MB/s.

Is there something about such large 'renaming' that actually results
in the file being really moved (aka copied) rather than just renamed?