So what exactly is coming with extended C++ support?

[Araq via Digitalmars-d]

>  It doesn't mention anything about moving C++ into C#.
> Even with IL2CPP, C# has fundamental design trade offs that 
> make it slower than C++(GC is just one of them), so it wouldn't 
> make much sense to port engine code to C# unless they wanted it 
> to run slower.

What are these fundamental design trade offs?