It is very rare for a BIOS change to affect the overall speed of a computer. Do you have a link to the knowledge base article that describes this fix? At any rate, the latest BIOS for the Dimension 1100 on the Dell page is indeed A01.
It is far more likely that the computers are running a fair number of background applications that get loaded when the computer is started. I know that every security program that I installed on my Dimension 2400 has taken its toll on the speed of my computer's operation. The one thing I know that will dramatically affect the speed of the computer is found in system setup. If you press F2 during the self test during a start-up to reach system setup you will find a menu item called CPU Information. CPU Information includes setting for the processor speed, compatible or normal. This setting should be normal; if it gets set to compatible the processor is run at a much slower speed. This should not be your problem; the default is normal and I have trouble believing that all 15 computers could somehow accidently be set to compatible.
Dimension 2400, 2.2 GHz, 640 MB, eVGA nVidia FX5200 128 MB
Inspiron 5160, 2.8 GHz, 512 MB, FX5200 Go 64 MB