Crosswind: Little Time Gain, Plenty of Risk
Strong wind feels fast, but it only produces time when it points the right way. Pure crosswind is the classic reason for disappointing segment times on windy days.
Mathematically close to zero
The component along your direction of travel is wind speed times the cosine of the angle. At 90 degrees the cosine is zero — the tailwind share disappears completely even though you feel 25 km/h of wind.
Yaw angle and apparent wind
The air hits you at an angle. At moderate yaw angles some deep wheels can generate a small forward force, but total drag from body and bike rises at the same time. Net, crosswind is almost always a disadvantage.
Control costs time
In side gusts you ride wider, correct more and leave your aero position. That is where the seconds go that physics alone does not explain. On open farm tracks with deep rims the effect is especially noticeable.
In short
Look at the angle to the segment, not the wind speed. Crosswind is noise, not signal.
Frequently asked questions
Are disc wheels faster in crosswind?
Aerodynamically often yes, practically only if you can keep the bike steady. Losing control is always slower.
When does crosswind become a safety issue?
From roughly 30 km/h gusts in open terrain. At that point a segment attempt is a bad idea both athletically and safety-wise.