I never liked the C#/Java comparison - Java was designed as a much higher level language, and allows reasoning about low level abstractions a lot less, while doing a ton more optimization in the JIT.
For example, GC escape analysis, automatic lock elision, devirtualization, tiered compilation are fairly recent features in C#, and likely not as mature/powerful.
Or C# has generic specialization, so if your generic param is a stuct, you get a separate implementation, while Java generics work via type deletion.
But in C# you have a ton more synchronization primitives, value types, methods are non-virtual by default etc.
This usually means that expertly crafted C# code can be faster than Java (and more importantly, you can trust the compiler to do the right thing), while if you wrote it exactly like Java, you'd probably end up with slower code.
For example, GC escape analysis, automatic lock elision, devirtualization, tiered compilation are fairly recent features in C#, and likely not as mature/powerful.
Or C# has generic specialization, so if your generic param is a stuct, you get a separate implementation, while Java generics work via type deletion.
But in C# you have a ton more synchronization primitives, value types, methods are non-virtual by default etc.
This usually means that expertly crafted C# code can be faster than Java (and more importantly, you can trust the compiler to do the right thing), while if you wrote it exactly like Java, you'd probably end up with slower code.