Apple announced that iOS 11.3 would allow users to see information about their battery health along with an option to manually turn off the slowdown.
If you have an older iPhone, you’ll want to check out your battery health. If you’ve updated to iOS 11.3, you can go to Settings → Battery → Battery Health (Beta).
This next screen will show your battery health. In this example, I’m using an iPhone X that is only a few months old, so my battery is still at peak performance. If your battery isn’t at peak performance, you can manually disable the slowdown.
Be forewarned; this might result in your device shutting off randomly if you are doing something that requires a lot of power. This option won’t be available until your first unexpected shutdown happens, though.