It can be many reasons, but I can assure you it's not K2 core. The problem may range from some plugin for K2 (or a Joomla system plugin) that connects to some external source which may time out to a badly configured server which under load performs poorly. And it can be both.

Since we specialize in improving the performance of any Joomla site, from the server's setup to the actual site's code, if you want to know the definitive reason why your backend is slow, just get in touch with us here: www.joomlaworks.net/support/get-help/contact