What subtle factors affect performance in unsubtle ways? As a non-professional biker who commutes on a bike daily I've been noticing that some factors that I didn't think would make much difference in how fast I go or how quickly I get tired have a much bigger impact.

For example, sideways wind. I did not think that a 5-10 mph wind from the side would slow me down as much as it does. Even tail wind doesn't seem to help.

Another is cold. I think that I can bike much faster when its 30C outside vs 0C. Its a little bit hard to say though because I can never compare them side by side.

This question may seem a bit vague, but what other factors affect performance in either positive or negative ways that one would not necessarily consider as making much of a difference?
Small changes in bike fit make dramatic differences in performance.  Heat and humidity make a big difference.