Hexxit lags if its not in full screen

[2PicksAndASword]

Hello, 

I have been playing hexxit a lot recently and am having a problem where if its not in fullscreen(minecraft's built-in full screen), I have display lag on my whole computer.

 

Computer Specs:

OS: OS X Yosemite

Processor: 2.66 GHz Intel Core i5 Quad Core

Memory: 16 GB 1067 MHz DDR3

Graphics: ATI Radeon HD 4850 512 MB

Computer: 27-inch iMac (2560 x 1440)

                     Second Display: SyncMaster Display 24-inch(1920 x 1200)

Storage: 726.47 GB free of 999.35 GB

 

I should not be lagging!

 

Help