Duplication speed KB/sec

You can actual see what speed duplicate are running at - indirectly.

The detailed part of the activity monitor will have a text like this:

25-08-2014 09:18:10 - begin reading
25-08-2014 09:31:48 - end reading; read time: 0:13:38
25-08-2014 09:31:48 - begin reading
25-08-2014 09:45:15 - end reading; read time: 0:13:27
25-08-2014 09:45:15 - begin reading
25-08-2014 09:58:58 - end reading; read time: 0:13:43
25-08-2014 09:58:58 - begin reading

In this scenarios we use a 50G fragment size (tape duplication) so dividing 50000/823 seconds (13:43)  = 60MB/sec. But as Marianne pointed out there are both read speed and writing speed when it comes to duplication.

And the writing of different backup images will vary in speed. Small images will show a lower speed than big images. Use it as a guideline