Thanks to the Battery Health settings that appeared in the latest beta, owners of iPhone 6 and iPhone 7 handsets will be able to disable performance throttling and to keep their devices running at full speed. However, iOS 11.3 has no launch date yet.
https://www.notebookcheck.net/New-beta-of-iOS-11-3-allows-you-to-disable-battery-performance-throttling.281710.0.html