It really depends on one's tolerance level. For some, the iPad mini 4 might deliver acceptable performance on iOS 11. For others, it may not.
Kinda like an unthrottled iPhone 6. Some might find replacing the battery on the iPhone 6 boosts performance sufficiently to handle iOS 11 well while others might consider it to be too slow, still.
Heck, I had to disable ProMotion on the iPad Pro 12.9 because after I used it for two weeks, the Pro 9.7 UI felt laggy (without installing any iOS updates or anything). It was entirely perception after getting used to the faster frame rates. With ProMotion off, the Pro 9.7 (and even the Air 2) feels just as fast as the Pro 12.9.