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If it makes you feel better, it could just be the weather. I mentioned I felt really sluggish yesterday, well I looked at the data and actually I put out quite a lot of power but was still slow. So, it's probably just a combination of road bike, lots of cold weather clothes, dense air and perhaps a more sensible application of the power, ie. not racing through possibly sketchy icy death corners, for example.
Maybe you're tired. Distance != speed