I stumbled across a performance issue for Outlook, which was really not easy to troubleshoot:
Some users, migrated to Exchange 2013, reported very poor performance in Outlook. Switching between e-mails or folders was just horrible. Sometimes they even couldn’t connect to Exchange and they got an error like this:
But not all users experienced this issue. I also had to mention we started seeing this first on Terminal Servers, where cached mode was not available.