How is this still an issue with version 15.20 (160315) which is current at the time of writing (April 3rd 2016). I have one of the most powerful personal computers it is possible to purchase, and yet scrolling large docuements (50,000 words) makes my Mac feel like a toy. 1-2 seconds lag on scrolling, crashes, ugh! The application is taking 87-100% CPU.
Go to Preferences
Spelling & Grammar
Uncheck "Check spelling as you type"
Uncheck "Check grammar as you type"
CPU drops to < 1%
Pathetic Microsoft, pathetic.