My MacBook Pro laptop running OS X 10.8.3 was performing abysmally. It was taking an inordinate amount of time to switch from application to application and to perform even simple tasks in programs. I closed VMWare, which helped, but then when I went to view a video in Safari I found I needed to increase the volume to hear the audio, but the pinwheel just kept spinning preventing me from doing so.
I started the Activity Monitor, which can be found in Applications/Utilities, to see what was happening. After starting the application, I clicked on the % CPU column header to order the process list by CPU utilization. The Activity Monitor was showing that the process SystemUIServer was using from 75% to 85% of the CPU cycles. I clicked on SystemUIServer to select the process then clicked on the Quit Process button then the Quit button to kill the process. The process got restarted shortly afterwards, but it was then using about 0% of the CPU cycles.
I found someone else experiencing the problem posting at High CPU use caused by SystemUIServer? that he found the problem to be asociated with menubar apps, specifically Dropbox in his case, but I don't use Dropbox and don't know what app may have triggered the problem on my laptop.