It's my opinion (being someone who has ridden both - even thought it's been about 4 years since i've ridden a geared mountain bike for more than a few minutes) that all things being equal - a rider on a fully rigid single speed is going to be faster than on a fully geared, fully suspended bike. Here's why I think this:
First and foremost the SS bike, due to its simplicity, has less moving parts, less friction, and (being fully rigid) will go exactly where you point your bike. And there is one other issue that many people don't consider. When you are riding the trails on a fully geared bike you shift a lot and many of those shifts will be going into much easier (slower) gears. With a single speed there is no shifting into easier gears...the gear is chosen specficially for the riders skill, strength, and stamina.
Now I understand that SSs spin out. And there are trails where that can be an issue (Otter Creek at times) but the right gearing mostly solves that.
Yes, I am a zealot. So chances of anyone convincing me otherwise are slim. But I generally find that most of the people I ride with (equally skilld riders as this argument will be) I am either leading the pack or on the wheel of the leader.
I have also noted that Stephanie has much faster on her SS than she was on the Palomino.
Single Speed bikes force you to be a better rider. Do they force you to be a faster rider? I think so.
What do you think?