The XHR requests are only made if you enabled the "Ultra" features inside the extension. By default, these are disabled and need a manual confirmation to enable them. These features include creating disposable email addresses, so the XHR requests are created at defined intervals to check if you received any new messages on these addresses.
Regarding resource consumption, it could be true. Extensions can behave differently on different systems. I would gladly investigate with you this issue, and the one with XHR, if you can provide me with more information that you might have by email at frustrations@haterapps.com
