What is happening when my Mac is being optimized after an OS X update? I have just updated to 10.9.2 and after the installation reboot this little message shows up:



What exactly is happening in the background here?
Here's how I would determine what's running when you see messages like that:


Check the battery icon if you have a portable.
Check if Spotlight has queued a large reindex from spotlight
Check Activity Monitor - look in the Energy tab and sort on Avg Energy Impact descending