In my experience with them, the performance hit is far more substantial that a few percent, except in situations where the compiler can elide the checks altogether. For example, simply iterating over std::vector with _ITERATOR_DEBUG_LEVEL=1 is twice as slow if you work with iterators explicitly instead of writing it as a range-for.

And I'm not sure if it can be substantially better, given that C++ as designed simply needs to do more checks to ensure validity - to catch cases like comparing iterators belonging to different containers, or iterators getting invalidated when containers get resized or when the corresponding element is deleted outright. This all can't be done with simple ranges or slices, which is why VC checked iterators maintain a reference to the parent container.