That is absolutely not true. Not historically, and not now. .NET's JIT has always been very good, bar a few qualifying statements.

It used to be based on MSVC, with the most expensive optimizations trimmed, things like loop hoisting, devirtualization, generally ones with long runtimes, or ones that required whole program visibility.

But otherwise it was competent, if you wrote code aware of these limitations, it could get very close to even C++, and even on naive code, it was always very competitive (with strange wildcards, that if you used raw pointers, which are an obscure C# feature, you could get code to be even faster, due to the compiler's heritage).

It also helped, that due to MS's nature; C# was meant to be used with MS frameworks, written by people who were aware of the compiler's quirks, which you could learn if you looked at these codebases.

Sure if yo measure against LLVM-compiled static languages, it falls short in certain scenarios, but it's been soldily in the pack of other GC languages like Java or Go, and always has been.