When is it more efficient to pass structs by value and when by ref in C#?

If you pass around structs by reference then they can be of any size. You are still dealing with a 8 (x64 assumed) byte pointer. For highest performance you need a CPU cache friendly design which is is called Data Driven Design.

Games often use a special Data Driven Design called Entity Component System. See the book Pro .NET Memory Management by Konrad Kokosa Chapter 14.

The basic idea is that you can update your game entities which are e.g. Movable, Car, Plane, ... share common properties like a position which is for all entities stored in a contigous array. If you need to increment the position of 1K entities you just need to lookup the array index of the position array of all entities and update them there. This provides the best possible data locality. If all would be stored in classes the CPU prefetcher would be lost by the many this pointers for each class instance.

See this Intel post about some reference architecture: https://software.intel.com/en-us/articles/get-started-with-the-unity-entity-component-system-ecs-c-sharp-job-system-and-burst-compiler

There are plenty of Entity Component Systems out there but so far I have seen none using ref structs as their main working data structure. The reason is that all popular ones are existing much longer than C# 7.2 where ref structs were introduced.

generally, passing by reference should be faster.
when you pass a struct by reference, you are only passing a pointer to the struct, which is only a 32/64 bit integer.
when you pass a struct by value, you need to copy the entire struct and then pass a pointer to the new copy.
unless the struct is very small, for example, an int, passing by reference is faster.

also, passing by value would increase the number of calls to the os for memory allocation and de-allocation, these calls are time-consuming as the os has to check a registry for available space.