The four privacy configurations I recommend are App Tracking Transparency, Mail Privacy Protection, Location Services, and Analytics sharing. Turning off tracking and mail monitoring significantly stops the collection of hidden information to refine ad targeting, while setting Location Services as "While Using" prevents an app from running in the background. Turning off Analytics and ad personalization will also limit unnecessary sharing. In my practice I have had clients notice less advertising, and greater privacy immediately after making these simple adjustments.
When I want to speed up my iPhone, I immediately do three settings. I first turn off Background App Refresh because too many apps are running in the background, which can slow down the performance. Second, turn down Motion and Transparency from the Accessibility settings because they look nice but use extra processing power. Third, I get rid of all the old widgets I don't use every day since they constantly refresh and slow down speed as well as battery. I originally started doing this before traveling long sourcing trips from Shenzhen, when I needed my phone to manage orders and calls to suppliers. It really makes a difference when you have lots of apps running on the phone, and it lasts the extra time too.