Update Safari V 9.0
Latest update of Safari (oct 2015?) for OS X Mavericks produced a tremendous slowdown of webbrowser: CPU of more than 100% of Safari-webmaterial and blocked. The extension 'translate' seemed to been the problem: after deleting this extension, and re-installing the newest listed in extensions list problem was solved.
iMac (21.5-inch Mid 2011), OS X Mavericks (10.9.5)