The slice type is an implicit struct, in the shape:

    struct {
        data uintptr  
        len  int      
        cap  int      
    }

Which is usable when the underlying memory is set to zero. So its zero value is really an empty slice. Most languages seem to have settled on empty slice, array, etc. as the initialized state just the same. I find it interesting you consider that the worst case.

Maps are similar, but have internal data structures that require initialization, thus cannot be reliably used when zeroed. This is probably not so much a performance optimization as convention. You see similar instances in the standard library. For example:

    var file os.File
    file.Read([]byte{}) // panics; file must be initialized first.