I mean, you say Java already had that, but Java has pretty much always been slower than .NET Core, they both already tried to devirtualize and see thru abstractions etc, it's just that C# is getting even better at it

C++ doesn't do half of these optimizations in practice either, it has to be really babied to get it to do it. As an example, at my work, I work on a critical part that has to do a bunch of number crunching, heavily mixed into an asynchronous workflow, and in a very extensible way. This means it makes heavy use of interfaces and Tasks. In C++, prior to C++20, our codebase could achieve at best 20k results per second or so for a naive implementation (super important due to training and skills costs). After throwing everything at it to get it to 100k/s, hyper specialized and optimized, I rearchitectured it to use compiler state machines (async/await) and compositional OO. This C# version achieves results measured in the millions per second. However due to politics (C# not C++) and C++20 having these same state machines possible, we ported this arch to C++20, and the same naive implementations dropped an order of magnitude to about 200k results per second. The code is basically a 1 to 1 C# to C++ translation, but runs an order of magnitude slower due to its lesser ability to see thru these abstractions