Difference in performance on iPhone 6s and iPhone 7 plus

I have an app, that is currently in private tests by “TestFlight”.

When I install an app from “TestFlight” app works fast on iPhone 6s, but when our client installs it from “TestFlight” on iPhone 7 Plus, the app works super slow.

That is surprising for mine because iPhone 7 Plus has better hardware. Probably I did something wrong on XCode build step.

Can you advise me what should I check?