Distinctive slowdown across browsers on 10.9.1

As a additional debug method I would try to create a new system user and see if the problem persists.

In the past I have similar problems after installing VPN software. Even using the uninstaller the system was sluggish.

The truth is, us power users, usually abuse the OS installing thinking tools, browser extensions, preferences panes, command line tools... Some are beta or even alpha.

If this is your case installing a fresh system on a empty drive is probably the best solution.

The issue is a failure of the Cisco VPN client, as described here, and it was fixed by running sudo /opt/cisco/anyconnect/bin/websecurity_uninstall.sh.

This had failed to occur to me because I hadn't used the client recently when the problem emerged. But looking through the console quickly revealed an absurd number of error messages that quickly identified the culprit.