Why does my Mac slow down when the volume/brightness indicator fades out?
What kind of OS X version that you are running? (Yosemite or El Capitan? I see in screenshot posted a volume/brightness indicator)

It sounds LIKE a good idea when you talking about Mac slow down when volume/brightness indicator fades out.

Here to fix this:

Go to System Preferences > Accessibility and then check "Increase Contrast" Option.

Also you can watch this video to fix about Slow Macs: https://www.youtube.com/watch?v=hi5VJsbRxzk

Same problem on my 2008 Mac Pro too, But i'm sorry that for didn't answer for a long, I am a first user using this forum, But People who don't know what problem this is.