If you live in a cold enough environment, like in Europe on winter, you'll eventually get it. If you live elsewhere, just go find a cold place. The performance is simply limited by the thermal throttling, which will happen when the device reaches certain temperature limit. So you simply need to find a place cold enough to keep the device from reaching that temp too fast. Otherwise, you can modify the thermal config to delay throttling until a higher temp value, but I don't recommend as you may turn Mi 5 into Note 7 bomb.