Please don't get mad at me for saying this, but if you haven't done so, take a minute and make absolutely sure your brakes aren't dragging. There is nothing that can sap your energy (and speed) like dragging brakes. They're worse than a headwind.

Having your saddle height/setback dramatically wrong could also affect your power, but you'd probably notice if your saddle was way off since you are used to how your old bike felt and the new one would feel immediately wrong.

When I went from a 52/42/32 triple with 11/23 cassette to a standard double with a 12/25 cassette, I did have an uncomfortable gearing spot that was somewhere around 42/11-13. 39/12 was a little easy (and rubbed on my bike) but 53/middle gears were a bit hard. Basically I had to get a little bit stronger so I could cruise in the big chainring.