There are many low-level devices where initialization is very expensive. It may mean that you need two passes through memory instead of one, making whatever code you are running twice as slow.

I would argue that these cases are pretty rare, and you could always get nominal performance with the __noinit hint, but I think this would seldomly even be needed.

If you have instances of zero-initialized structs where you set individual fields after the initialization, all modern compiler will elide the dead stores in the the typical cases already anyway, and data of relevant size that is supposed to stay uninitialized for long is rare and a bit of an anti-pattern in my opinion anyway.

Ok, those developers can use a compiler flag. We need defaults that work better for the vast majority.

meh, the compiler can almost always eliminate the spurious default initialization because it can prove that first use is the variable being set by the real initialization. The only time the redundant initialization will be emitted by an optimizing compiler is when it can't prove its redundant.

I think the better reason to not default initialize as a part of the language syntax is that it hides bugs.

If the developers intent is that the correct initial state is 0 they should just explicitly initialize to zero. If they haven't, then they must intend that the correct initial state is the dynamic one in their code and the compiler silently slipping in a 0 in cases the programmer overlooked is a missed opportunity to detect a bug due to the programmer under-specifying the program.