Remind me again... the Downtube 8FS is a full suspension bike? If so, the suspension will also absorb some energy and make a bike feel a little more sluggish. Another important thing is... tires. Tires can make a significant difference.

If you search the forum you'll find some great discussions on wheel size and what not. I'm not expert enough to say whether it makes a difference, but I've found the discussions both illuminating and entertaining.

--sam