Why is my late 2013 iMac so slow?
i've seen this on some of my machines... it is fixable (though ican't remember the details for my situation). but it will stay very slow until you clean out those ui/windowserver errors.

firstly:
have you added any screensavers?
have you safe booted to clean out caches and check HD?

http://www.macworld.com/article/1163410/bugs_and_fixes_when_os_x_lion_slows_to_a_crawl.html