Listen to the rpenmanparker's comments here, I doubt geometry is the issue, the bikes are too similar. It is likely materials. Please bear in mind that feel is very subjective and changes from day to day. Put your old wheels on the bike. Also note that Cannondale worked on the *Six* for a long time. They know what works on that bike and nearly all versions of the Six were highly praised for it's road feel. Road feel does a lot for the perception of speed. Ride a couple of more rides on routes where you know you have ridden the Six13 well and see if your overall times improve.

Problem you will have is that, a certain level of bike/rider most bikes behave and perform the same way, and its not like you went from a Denali to the Scott, Your reference is a very well balanced performance bike in the Six13. There will likely only be marginal differences in how it handles and performs.