How to determine whether their iPhone is down-limit performance?

Conditions to be satisfied at the same time:

1, in the model: iPhone 6, iPhone 6S, iPhone SE and iPhone 7;

2, iPhone 6 / iPhone 6S / iPhone SE software version is in iOS 10.2.1 and higher; iPhone 7 requires iOS 11.2 and above;

3, cold external temperature or full battery capacity has less than 50% of the original capacity of 60%. For the latter, you need to download a tripartite application view, such as Battery Life, Battery HD + and so on.

If you are "lucky" to fit into one of the three states of 1,2,3, then the basics are that proper performance is limited.