"Assuming it takes an average of 5 seconds per interaction with a cookie banner".
People don't spend 5 seconds clicking accept. They start reading their website, notice the banner in their periphery shortly after, and click it to go away.
The banner often obscures the view, and the user need to dismiss it in order to know if the page contains the information user looks for. But in order to dismiss it, you can click "Accept all" easily and job done, OR you need to enter options by clicking the "manage" button, find your way around the banner, understand what are the options and which ones are checked and which are unchecked, then click "consent" or whatever terminology is used, but only after finding the button.
This easily can take up to a minute.
Yes, sometimes there's the "reject all" button, but not always. Also you need to know how they define the "reject all" button, because sometimes it probably may be defined as "reject some".
It's not a problem for me, because I autoclick them away with browser plugins, but when I'm on a mobile, the web is really hard to browse because of those banners.