The modern cryengine compiles very fast. Their trick is that they have architected everything to go through interfaces that are on very thin headers, and thus their headers end very light and they dont compile the class properties over and over. But its a shame we need to do tricks like this for compile speed as they harm runtime performance.
Because you now need to go through virtual calls on functions that dont really need to be virtual, which means the possible cache miss from loading the virtual function from vtable, and then the impossibility of them being inlined. For example they have a ITexture interface with a function like virtual GetSize(). If it wasnt all through virtuals, that size would just be a vec2 in the class and then its a simple load that gets inlined.
At least on clang with LTO, with bitcode variant, that should be possible to devirtualize, assuming most of those interfaces only have a single implementation.
In my experience, as long as there's only a single implementation, devirtualization works well, and can even inline the functions. But you need to pass something along the lines of "-fwhole-program-vtables -fstrict-vtable-pointer" + LTO. Of course the vtable pointer is still present in the object. So I personally only use the aforementioned "thin headers" at a system level (IRenderer), rather than for each individual object (ITexture).
In addition to what everyone else has said it also makes it difficult to allocate the type on the stack. Even if you do allow it you'll at least need a probe.
We didn't create this code base ourselves, we are just working with it. I'd assume the original developers payed attention to compile times during development and introduced forward declarations whenever things got out of hand.
My computer is fast, AMD Ryzen 9 7950X, code is stored on an NVMe SSD. But there certainly are projects with fewer lines of code that take substantially longer to compile.
