There are many reasons that make your iPhone 6 Plus running slow. In my opinion, the main reasons of your iPhone 6 Plus getting slow are full storage, low battery health, and lack of system update. Here I will introduce some methods to fix this issue effectively.
1. Full storage. Too many cache and cookies, and big files may fill up your iPhone storage. For cache cleanup, you can head to "Settings > Safari > Clear History and Data" to clear cookies on Safari. For other third-party apps, you can uninstall them and then re-install to clear cookies. For big files, you can transfer those files to computer or upload them to iCloud to free up more space on your iPhone.
2. Low battery health. Navigate to "Settings > Battery > Battery Health" to check your iPhone maximum battery capacity. If it is lower than 80%, you need to change a new battery. Because Apple will limit the performance of low battery health iPhone to make sure the long battery life.
3. Old system. Newer system will improve the iPhone performance in some aspects. You can go to "Settings > General > Software Update > Download and Install" to check and update your iPhone to the latest system.
For other reasons and solutions, read on:
https://www.leawo.org/tutorial/how-to-fix-iphone-running-slow-1409.html?cleartalking=1409