Are they stock bikes? Friction shifting the Cannondale vs. Index on the Giant MIGHT have you in the less optimal gear riding the Cannondale in a fast group. You need to put nice tires on both bikes to compare. Are saddle, pedals, and bars in the same place relative to each other and the dropouts? My 86 Cannondale is as fast as (or faster than) any of my other bikes.
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I don't do: disks, tubeless, e-shifting, or bead head nymphs.