Locally--so the speed depends on your CPU. 10 GB/hr is likely.

The unique SHA-1 identifier is being calculated for the file, and the results are stored in the filurns.cache. Once done, it is reloaded on subsequent startups, so the process doesn't need to be repeated.

The hash guarantees that the file sent is the file received. If a corrupt file is hashed, that will guarantee a corrupt file is received. So, don't share corrupt files. If you do have a known file, you can use the Bitzi lookup features (right-click on a filename in the search results or the Library) for more info.