I was taking pictures of dancing with my N90s in a rather dark place once and saw another fellow with D70. I asked him if I could try his camera. My first impression was that the viewfinder of D70 was much brighter but focusing speed was much slower than that of my N90s, almost as slow as that of digital P&S's. Maybe I did something wrong; it was the first time I held D70 in my hands. I also must say that I had a flash with an AF-assist light on my camera.
Could be if the motors in each body driving the screwdrivers are different power or run at different speeds.
The CAM module may be the same but that only affects the speed of decision-making (ie in- or out-of-focus, direction to drive etc), once the decision is made then the physics of the drivetrain come into play, motor speed/power and gearing of the lens focussing mechanism.
Although the F100, N90, etc do have a faster screw drive mechanism, the type of lens must be taken into consideration also. The AFS lenses (such as the 18-70 kit lens) do not rely on the screw drive, rather they use the silent wave motor. In this case, the CAM module would be the decisive factor and not the turns of the screw. That said, I do not know which lenses were being used at the time.
I have shot jousting and Indian Ceremonial dancers inside a gym with the 80-200 f2.8 D and it works fine. The 28-105 works for running horses as does the 18-70. The D70 will miss about 5% of the dancers. Outside it works fine.
The D70 will follow horses running out side with the 80-200 f2.8.
The F100 is much better and the D2x which I do not own is also better.
Put the focus on C once it locks on it will follow focus fine.
The camera has trouble tracking brides coming down a dark isle as the side sensors do not lock on as well as the center sensor.
Original poster noted he had a speedlight with focus assist on. Don't think the D70(s) built-in focus assist is nearly as powerful as a flash unit's focus assist (at least, it doesn't appear as strong as my SB800's).
Also, may have a been a difference between focus priority - the D70 may/may not have been set to either AF-S or AF-C. AF-S places focus priority above shutter release; AF-C provides continuous focus and places shutter release over focus priority.
A combination of AF-S and lack of either a speedlight focus assist (or the body's focus assist may have been turned-off) may have - at least from a perceptual basis - indicated slower focusing.
In short, don't doubt your perception, but there may have been some variables that accounted for differences. If you're contemplating a D70(s), you may want to do a side-by-side comparison in a shop or with a friend.
Take your lens off the camera and look at the mount end. Count how many turns the screwdriver makes while the focus moves from one extreme to the other, from memory the Nikon 28-105 is about 4 turns, the Nikon 70-300 G about 14 turns. This gearing will drastically affect how well the camera can follow a target and how quickly it will focus (although it may depend how out-of-focus it was when you pressed the shutter button).
The Pro cameras such as the F100 etc have the ability to turn the screwdriver faster than the prosumer bodies, good quality glass such as the 28-105 and 80-200 will have better gearing as above, (AF-S even quicker), however mount a 70-300G on an F100 and it can still take a while to focus....