If the Mac lags after the update to Mac OS X 10.10 "Yosemite", it does not have to be due to hardware that is too old or too little RAM: With my 2011 iMac, which was really fast with quad-core i5 and 16 gigabytes of RAM under "Mavericks". After the update, the "Beachball of Dead" often appeared for minutes after the system start and later during operation - the beachball of death because the system load was so high. If you should have the same problem: The obviously faulty cloudd process is to blame. We'll show you how to get your Mac up and running again immediately. The activity monitor helps with this.
Lame Mac after update: Cloudd is the culprit
First open the activity monitor from /Applications/Utilities/ and sort the processes according to their processor load or processor time. If you've been struggling with the problem for a while, you'll find the "Cloudd" process here, a system process that is responsible for synchronizing the computer and the data in iCloud. This constantly eats up well over 20% of the processor time, with me it was even constant 91 percent!
The cause: a bug!
The cause seems to be a bug in the process, and there are sometimes incorrect settings after updates. Therefore, open the folder ~/Library/Application Support/ and move the "CloudDocs" folder you find there to the trash can. Then use the activity monitor to kill the “bird” and “cloudd” processes. Your system load should now be significantly reduced on the spot and the Mac should be up and running again. If the problem occurs again after a restart (a few minutes after the start is OK!), you can repeat this tip, you will not initially suffer any technical disadvantages.
Alternative: switch off document sync
If you don't know how to delete files from the Yosemite library or you prefer not to destroy unnecessary files, you should try the following trick: Open the system settings and there the "iCloud" menu. Click on the "Options" switch next to "iCloud Drive" and deselect all programs that you don't use or don't use with iCloud, ideally all of them. This also significantly reduces the system load of the process. It is to be hoped that Apple will fix the bug with the next update.