Site incredibly slow for logged in users/admins, but normal on frontend

Hi all,

For the past several days, we’ve been having some trouble with our website. It works fine if you view it as a non-logged in user, but if you’re logged in as a writer/admin every page (including the backend) takes ca. 2 minutes to load, and occasionally returns a database connection error rather than loading. We’ve been in touch with our host, who has informed us that the server and the database look fine on their end. We’ve cleared some of our disk, to no avail. We have not installed or updated plugins, or updated WordPress, in a while so that isn’t the issue. This very much started out of the blue. Does anyone have any idea what might be going on?

The page I need help with: [log in to see the link]