A violent bouncing on the suspension at high speeds is what the driver feels. The cause is an aerodynamic one, where either the leading edge of the floor, or perhaps the front wing, is pushed ever closer to the ground as the downforce acting upon it increases. The closer to the ground it gets, the more powerful the ground effect is, as the air rushes ever-faster through the shrinking gap.
Sauce: https://www.formula1.com/en/latest/...ams-a-headache-at.6O93LlvFMyY81L9LfSutcS.html