Beam Sync (forced VSync in Yosemite)
Mac OS uses something called "Beam Sync" which is basically just VSync. it stops screen taring when using the UI (moving around windows, switching spaces, etc.). This wasn't an issue (except for some Hackintoshes that didn't support it) until Apple became much more strict about forcing it on in OS X 10.10 Yosemite. Beam Sync can cause huge input lag. It really feels like your crosshair is dragging through mud, especially if your FPS isn't great. Luckily there are still two ways to disable it in Yosemite (using Secrets no longer works). Each with their own advantages and disadvantages
Method 1: Download BeamOff[www.insanelymac.com
When you run BeamOff it disables Beam Sync and then closes itself. If you run it before playing CS:GO, you will notice that the difference is night and day. The crosshair will feel much more responsive and the input lag caused by Beam Sync will disappear.
The disadvantage is that, since it disables Beam Sync, it can cause noticable screen taring in other things. If this doesn't bother you, go right ahead and put it in your login items (in the "Users and Groups" preference pane) to have it load when you login so you don't have to worry about it. Otherwise simply start it before you want to play.
Unfortunately I haven't found a way to easily re-enable Beam Sync using this method aside from rebooting (or possibly logging out and back in again).
Note: It is normal that BeamOff exits instantly. It makes the changes and then closes without any feedback. Rebooting resets it.
Method 2: Download Quartz Debug (you will need to have a free developer account and Xcode installed)
Start Xcode and go to "Xcode" in the menu bar and then "Open Developer Tool" > "More Developer Tools...". It will take you to the Apple Developer download page. Log in with your developer account and download and install "Graphics Tools for Xcode", which contains Quartz Debug.
Once you have done that open Quartz Debug and set Beam Sync to "Disable". This setting only stays active while Quartz Debug is running, so keep it running in the background while you play.
By the way, if you used method 1 (BeamOff), you can go into Quartz Debug and then set Beam Sync to "Disable" and then back to "Automatic" to re-enable Beam Sync.