What is x-speed-cache-key? What is its purpose?
Best I can find on Google is that this header (and its companion header 'X-Speed-Cache') are added by a web server level module called "Speed Booster" touted as part of the Drupal Platform 'BOA': https://groups.drupal.org/node/173219 and https://omega8.cc/features . 

The purpose of the header seems to be nothing more than a debug header to check which key has been used to retrieve the cached version in Speed Booster.

You probably want to contact your hosting company to see if they can disable that header.