Apple - What is happening when my Mac is being optimized after an OS X update?

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

It's Spotlight reindexing. The same notification appears after OS X 10.8.x → 10.9 update, as well as after all 10.9.x updates so far.