I'm using Calibre on a Windows PC. My readers are the Kindle PPW, iPad and iPhone apps.
Anyway, I was doing this over my laptop with a mobile connection, which isn't all that fast. I tested it on my PC which has a fast ADSL2+ connection. The ebook was sent in about 30 seconds.
Can't work out though, why there is a 5 minute discrepancy between the two. The mobile connection isn't that slow, as Outlook can send emails with attachments, Dropbox receives files and so on with no noticable delay.
Whatever, the problem isn't with Calibre. I was merely curious as to what others were experiencing.