Some ideas from
https://www.imobie.com/iphone-tips/speed-up-slow-iphone-ipad.htm:
"You might have opened too many apps and games on your iOS devices. So just double click Home button and slide to shut down the apps/games you are not using to free up memory on your iPhone or iPad."
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"Do you remember the last time you organized your Safari history and bookmarks? Actually, we almost never organize our Safari info and clean browsing junks. That’s partly why our iOS 13 goes slow when we surf online by using Safari.
To Organize Bookmarks and History
It’s easy to organize your browser bookmarks and history. Just run your Safari app and click Book icon, you will find all of your history and bookmarks there. You can delete any history or bookmark items, or edit your bookmarks.
To Delete All Browsing Data and History
Go to Settings > Choose Safari > Tap on Clear History and Website Data.
To Delete All Browsing Data of One Specific Website
Go to Settings > Choose Safari > Choose Advanced > Tap on Website Data.
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"Background apps activities are often the culprit slowing down your iOS operating system. Luckily, Apple offers its users the features to turn off background refresh and auto-update to make their iPhone/iPad have a good performance.
Speed up iPhone/iPad – Disable Background Apps Refresh
Speed up iPhone/iPad – Disable Background Apps Refresh
How to turn off background apps refresh
Setting > General > Turn off Background App Refresh.
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And of course, you can try a hard boot...hold down the power and the home button until it powers down, and then restart things after it has shut off.