This is more a theoretical question: does the use of block-elements reduce page-speed or increase it? I ask this out of curiosity. I would guess: pages will load faster? Thanks for taking time.
Adding anything to your pages that wasn’t there previously will decrease your page speed. This is because it’s something extra the browser has to download as the page is rendered.
The more complex the element you’re adding, the more of an effect it will have.
Of course, in most cases, this effect should be very minimal as long as there aren’t massive images to download etc..