What are the Disadvantages of Nested Vectors? (C++)

I'm still fairly new to C++ and have a lot left to learn, but something that I've become quite attached to recently is using nested (multidimensional) vectors. So I may typically end up with something like this:

std::vector<std::vector<std::string> > table;

Which I can then easily access elements of like this:

std::string data = table[3][5];

However, recently I've been getting the impression that it's better (in terms of performance) to have a single-dimensional vector and then just use "index arithmetic" to access elements correspondingly. I assume this performance impact is significant for much larger or higher dimensional vectors, but I honestly have no idea and haven't been able to find much information about it so far.

While, intuitively, it kind of makes sense that a single vector would have better performance than a a higher dimensional one, I honestly don't understand the actual reasons why. Furthermore, if I were to just use single-dimensional vectors, I would lose the intuitive syntax I have for accessing elements of multidimensional ones. So here are my questions:

Why are multidimensional vectors inefficient? If I were to only use a single-dimensional vector instead (to represent data in higher dimensions), what would be the best, most intuitive way to access its elements?