Is upgrading to iOS 5 faster on a Mac?
For what it's worth, iTunes is probably exactly the most bloated and slow software that Apple includes on a Mac, as it's pretty much the only built in app that isn't 64 bit, and until very recently it wasn't even using Cocoa but Carbon APIs which Apple have been telling people not to use for years now.

Having said that, the largest bottleneck is the speed of the USB transfer along with the ability of the iphone to process the update and write to its flash memory and validate it etc, so really I wouldn't expect to see a massive difference.