There isn't a lot you can do. At five years your iPad is near the end of its supported life cycle, and the advances in iOS and the nature of the web are pushing it to its limits. However, you can try resetting it. This may clear up any accumulated problems.
First do a good backup. Also look over anything that may be stored locally (not in the cloud) and save it do another device or computer. Then go to Settings > General > Reset > Erase All Contents and Settings. When prompted, restore from the backup.
More extreme, you could choose not to restore from a backup. If all you use the iPad for is a few things, and you don't care about what is stored on it, a clean reset is the closest you can get to optimizing your iPad. You'll then have to re-install any apps you want and re-add any services. iCloud content should recover as long as you use the same iCloud account during the reset procedure.
In both cases, you risk losing content, so be sure it's in the backup or saved elsewhere.
There is no guarantee this will improve the iPad's performance.