Method dispatch using a vtable (a pointer, embedded in the object, pointing to a record of methods) is way faster than practically any other form of dynamic dispatch (except calls of known functions, of course). To support it, you pretty much need some form of single inheritance/subtyping (C++ does support multiple inheritance, but it significantly complicates the implementation). Rust's single-dispatch type-classes and Go's interfaces are dispatched in a similar way, but the record of functions is passed along the pointer to object as a "fat pointer". Servo people figured out that that is too wasteful/slow compared to C++-style vtable approach.