Usually renderers do a z pre-pass to eliminate overdraw. Real reason for deferred shading to be faster would be lights. In normal forward, every light has to be accumulated for every pixel. And this problem is solved today with clustered shading (aka forward+). Link to paper: https://www.cse.chalmers.se/~uffe/clustered_shading_preprint.pdf

In short, these days deferred shading is slower because of memory bandwidth.