Show download speeds in Safari
After reading this thread and with the reproducibility described here, I started to believe that this is a bug. In this thread OS X users complain that sometimes download speed is visible and sometimes not.
It was suggested to completely quit Safari, open terminal and execute:

rm ~/Library/Safari/Downloads.plist


And launch Safari again. That did not work for me.

The following steps might show your download speed (80% change, your mileage may vary):


Start a download
Pause that download
Close the download window
Wait 5 seconds and open the download window
Resuming the download
(after closing the download window, you need to do this all over again)


Screenshot, showing visible download speed (OS X 10.8.5 and Safari 6.0.5):