WHAT IS IT?
Do this monthly to speed up your phone - Clear your smartphone cache. Safari and other browsers explained in the link
Think of your browser cache as a digital storage closet. Over time, an iPhone stores website data like photos and banners in that closet to retrieve them when you visit a site. Most of the time, this speeds up your browsing. Before too long, though, that closet gets cluttered, and the contents may no longer match what you need.
Here's what to do.
1. Open the Settings app on your iPhone.
2. Select Safari from the list of apps.
3. Go to Advanced > Website Data.
4. Scroll down and select Remove All Website Data.
5. Choose Remove Now in the pop-up box.