iPad mini 4 has a regular A8 chip. iPad Air 2 has a Tri-core A8X chip with (more importantly) an enhanced GPU that actually can handle the blurring effects and animations perfectly smoothly. For single core, iPad Air 2 has about a 15% edge in performance. For multi core, a whopping 60% edge.
The GPU is DRAMATICALLY better with the Air 2. I was using an iPad mini 4 in store and was not impressed. A lot of janky, slow, choppy animations JUST like my iPad mini 2. iPad Air 2? NONE of these issues. At all. Butter smooth, super snappy, perfect.
A lot of other things such as L2 Cache, memory performance/bandwidth, etc etc are also better on the Air 2.