Reeelllaaaaaxxxx.
That is the MOST important thing to do in that kind of wind.
Do NOT hold your arms or upper body stiff. Let the bike move around and only counter the bigger moments.
Grip the tank with your knees and move around on the bike if you have too, but don't use your arms to brace against the wind. Your body acts as a sail and that will transmit steering input through your arms to the bike. Stay relaxed.
As Merv said speed isn't the issue, though going too slow can make things much worse. Your wheels give you a gyro effect thanks to rotational inertia.
If you find yourself being blown wide on a corner STAY ON THE GAS. Bikes steer much better on the gas. Use countersteering at the bars to adjust your line, don't panic, button off, and try to get upright, or you WILL end up on the wrong side of the road.
Stay relaxed mentally too. Try not to approach a windy day in a state of panic.
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