Performance and loading times of Zen vs building a theme from scratch?

Ive used Zen for every Drupal site ive made for years so im well aware of the benefits and how much it speeds up front end development. However I have noticed that there are far more div tags and css than would be needed if I built a theme from scratch and only included exactly what was needed. Especially as my clients are increasingly interested in mobile optimisation, are there any stats or anecdotal evidence about how much of a performance hit this extra markup is causing?

Thanks