Why does WordPress pre-load an un-requested page every time it delivers a page?

That doesn’t sound right at all. Have you tried:

– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).

– switching to the default theme to rule out any theme-specific problems.

– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.

Odd but true. Just set up a default WordPress install here.

WordPress starts pre-loading additional pages if you use anything other than the default permalink setting. The impact on page delivery time is is substantial.

I’m confused. Why would this be so.

I’m not seeing any un-requested page loading. Have you tried using another browser? Or another computer?

Are you sure?
Have a look at this screen grab

I would imagine this is caused by your browser prefetching URLs (more info: https://www.ghacks.net/2013/04/27/firefox-prefetching-what-you-need-to-know/)

Thanks esmi and Scott. Still a little unclear. Why does it only prefetch URLs when search engine friendly permalinks are turned on? To help enhance search engine ranking in some way?

Your browser may be grabbing the info from your canoncial meta tags (https://googlewebmastercentral.blogspot.co.uk/2011/09/pagination-with-relnext-and-relprev.html). Its possible these are only being output when search engine friendly permalinks are being used.

Look for code like this in the head of your page:

<link rel='prev' title='title' href='https://url.com/post-title/' />

You may have more luck if direct your question at either the browser or extension that is doing the prefetching.