Apple included this tweak after backlash in late 2017, when users complained that they noticed their phones slowing down. It turns out that Apple's software was automatically limiting performance during times when the phone was drawing peak power from the battery, in an effort to preserve battery life and avoid sudden iPhone shutdowns.
Now, you can make this choice yourself: with iOS 11.3, you can see the health of your battery and decide whether or not to sacrifice battery life for power.
- Make sure you have iOS 11.3 installed. Go to Settings > General > Software update.
- Go to Settings > Battery
You'll see this menu. Click on Battery Health (Beta):