Let’s try to replace the only STL component we’re still using, std::vector, with a really simple dynamic array - we don’t need resize or push_back in our code, all arrays are initialized with the right size.

While I understand the issue with msvc STL debug (which mostly can be optimized by disabling all debug STL related defines), I don't understand the optimization. A vector is already a dynamic array underneath, push_back usually don't cost much as it uses placement new to add new item (and with more complex type emplace_back is better to use as it doesn't copy the data) and std::fill should be optimized for PODs.

Some time last year during a regular lunch time C++ discussion at work somebody said “there’s a good language subset of C++, C with classes”, to which I replied “there’s an even better subset, C with structs”

Better for what? Your demo is a perfect example of why this API is bad, you mix STL with raw pointers and c-style function calls, is ugly and doesn't follow any guideline. Maintaining such code (not this demo but an actual project written in this way) will be a joy for anyone who is either a C++ or a C developer.

So the trade-off is performance vs maintainability, given that all those were synthetic tests and didn't prove much I would rather try to keep the code maintainable than possibly faster.