> In C# you can make data structures which [...]

Yeah but that's completely validating my point. C# is not Python or JS. It's a (remote) cousin of C which tries to take some of the valuable performance tools from C and bring those to a managed runtime. Because it's strongly typed, it's a lot easier for the compiler to optimize, and because you have all these tools to design compact objects without pointer, you can do that job so the compiler doesn't have to.

And again, an experienced C# programmer can probably write code that runs circles performance-wise around code written by an experienced JS/Python developer in most cases.

> JIT runtimes have more input data than a C compiler

They have more data, but they are at a disadvantage by being time-pressured. They can't apply costly analysis to these data because they need to compile fast. Therefore typically they limit themselves to local analysis which may miss a lot of opportunity for inlining.

Actually, action for action managed languages tend to be "faster" than C in the sense that writing the equivalent program would be slower in C. However, the additional freedom leads to increased program complexity and that complexity eats into the performance budget enough to end up slower than "idiomatically" written C where the developer distilled the solution to its bare essence.

Javascript has the fastest general purpose hashmap/dictionary implementation of all programming languages but at the same time you are forced to use them all the time which overall slows the language down. Writing exactly the same code in C would be even slower since C hashmaps aren't as optimized. However, C code is rarely written like that so it's usually faster.

I don't know about LLVM but GCC has had something like this for years as a standard optimization feature. You create a binary with special profiling code which writes to a file. After running the program a few times, you recompile with optimization that uses this file. I forgot the flags though.

Personally, I'm more interested in executable optimization. It decompiles an executable, performs whole-program optimization, and re-compiles it. I'd love to tinker around with optimizing other people's binaries (e.g. games) for my machine. There is something like that for LLVM but it's very experimental.