Blood Legion

So I play a feral druid and I have been wowarmory-stalking you to see how you have been gearing and reforging to and I noticed you switch between mastery and haste very often. What is the reasoning behind that or are the any percent caps I should be looking to hit. I have heard that once you get the 4 piece tier 13 you are supposed to go haste, but why? My guild is progressing through DS right now and I figured you would be able to help up my dps by answering some questions. Any help you could give would be amazing.

Thanks for your time!