What exactly is slow about std::vector? [duplicate]

This question already has an answer here:

• Is std::vector so much slower than plain arrays? 17 answers

Don't rush to close this as opinion based, for I believe there must be real experience behind people saying so.

Having spent 2 years at a CS department, I've heard quite a few times from people doing computer vision, graphics, various physical and economical computations that

std::vector is slow. When you need speed, go with a plain array.

What exactly is so slow about std::vector? What has remained slow, if anything, with the introduction of data()that gives direct access to the underlying array in C++11?