It's because it's trivial to improve. The reason the MSVC STL is so slow in debug mode, is because it's really debug; they pull out all the stops to bounds-check every dereference, keep a generational count to detect iterators that could potentially have been invalidated by an operation that moved elements or reallocates the container (and do so whether or not the potential resize actually happened), etc. This adds a ton of code to the iterator paths, and (due to that code size) loses most inlining opportunities.

If you want it to be as fast as other implementations, ask it to do less of this. They provide [_ITERATOR_DEBUG_LEVEL](https://docs.microsoft.com/en-us/cpp/standard-library/iterat...) for exactly that purpose. I think their choice of default (Debug means you want it to catch as many bugs as it can, and Release means it should be fast) is appropriate.

If they hadn't provided the knob to adjust this I'd agree they had a problem for some use-cases, but if you want the faster-but-less-thorough mode you just have to ask for it.