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.
You'll see this menu. Click on Battery Health (Beta):
Now you'll see this page:
As you can see, my iPhone X still has 98% of its battery health and is running at optimal performance.
If you have an older iPhone, you may see a lower battery health percentage. That's because batteries begin to deteriorate and lose their ability to hold a charge over time. If it's below 80 percent, you'll see a note that performance is being limited as a result.
This means your iPhone is purposely saving power so that the battery doesn't automatically shut down when you don't expect it to.
If this is the case, Apple will show an option to disable the performance throttling. Tap "disable" if you want your iPhone to run at peak performance, but at the cost of battery life. Just understand that your phone may shut down suddenly, or sooner than you expected, as a result.