How to soften twists and turns to change the speed during movement?

Can you give some context? What is your current vcam setup? What is your target? How is the target moving? What is the camera doing now and what would you like it to do instead? Showing the vcam inspector would be helpful.

 

(you gave the same screenshot twice, probably by mistake )

Do you use a dolly cart to move both the target and the camera? The cart will move at the speed you set in the inspector. You have to change that field smoothly in order to change the speed.

Is it necessary to have the camera on a dolly track? It might be easier to make a virtual camera that just tracks the car without being on a dolly track. Can you show the inspector for the virtual camera?

 

You would need to add some logic that sets speed according to path position. Perhaps a custom behaviour with and array of items { path position, speed }. It monitors the current path position, and updates the speed according to its data.