I think the main external factor is the thinner summer air (higher temperature and humidity). But I think it's more than that, depending on the individual. Somehow, perhaps because it's harder to breath the cold dry air, or harder to warm up my circulation, I just don't put out as much power.

Ice and snow are a whole different story. I don't even think about speed on ice.

btw, I'm not answering the nominal question about speed because it just won't convey any meaningful information. There are too many variables between the environments of your speed and of mine, or almost anyone else's, for there to be any sort of comparison. The drop in speed for me, in the Georgia "winter", seems to be about 10% or sometimes more.