This applies to non-GC languages as well. Memory management is slow. Even with manual memory management I have been able to dramatically speed up code simply by modifying how memory is allocated.

Parts of the GC language crowd in particular have come to hold some false optimistic beliefs about how well a GC can handle allocations. Also, Java and C# can sneak in silly heap allocations in the wrong places (e.g. autoboxing). So there is a tendency for programs to overload the GC with avoidable work.

Autoboxing is more a Java problem mainly because of type erasure with generics. C# has "proper" generics and no hidden boxing is occuring there.