Speed up OS X Yosemite
Turn OFF Transparency
This guide will help you get your Mac, that is now runnings OS X Yosemite, back to a like-new performance so you could enjoy Yosemite's new features in peace. The key to getting rid of the lag is turning off the Transparency feature. Yosemite won't look as pretty, but it will do the job.
Step 1: Click the Apple logo on the top left corner of your screen
Step 2: Click "System Preferences"
Step 3: Click "Accessibility" -- (The blue circle with the white stick figure)
Step 4: Click "Reduce Transparency"
Now you will enjoy Mavericks-like performance, while enjoying all of the new and fantastic features in Yosemite. As you will see below, it is a noticeable change, but I am sure that the performance difference is worth it.