The average time is not a good measure on your performance, it's an average number of the units you do since you started. The only thing it realy shows if your PCs are fairly new or old.
The most interesting number is the amount of WU's you do per day, the more PCs, the more WU's.
There are two ways to get a good number of WU/day calculation:
1. Check your stats in 24hr and see how many units you increased (for more accurate make an average of 2-3 days).
2. If the PCs are all in the same building and connected over a LAN then use SETIWatch that will keep track of all your PCs and report all the units and count averages etc.
I use SETIWatch at home and it's very easy to see how fast your PCs are and their individual averages. You can also track fast and slow WUs and find out why some are faster than others, etc.
Download it here!
http/members.home.net/mloukko/SETI.html