how do i downgrade my ipad mini from ios 8.1.3 to ios 7.1.2

You don't. You cannot downgrade iOS levels.

What specifically is your performance issue? I haven't really seen any difference between iOS 7 and iOS 8.

Here are some general performance suggestions from the Internet:

http://www.computerworld.com/article/2833834/how-to-improve-ios-8-performance-on -your-iphone-or-ipad.html

http://osxdaily.com/2014/10/12/fix-ios-8-slow-performance/

Since Apple does not support downgrading the iOS I suggest you start a new question that gives a good title to a specific problem. Then in the body of the thread describe the problem in detail, tell us what steps you have tried so far to correct the problem, what happened, and what error messages you received.