What weight oil are you using? Stuff like 10w-30 when below freezing will get sludgy. When you start up in the morning the oil pump has to try and pump sludge thru the engine and with sludge on each component it touches, it adds more parasitic loss to the overall performance. When the oil heats up it flows very nicely, enough so that you notice it running slightly better. That's my guess, unless you actually have something wrong on the engine (say temp sensor or something with the a/f mix when cold).
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