As your shop lives, some items of PrestaShop can grow very big in time. If you notice your shop is getting slow the following list of items can be analyzed to see if you can clean it.
Some SQL tables of PrestaShop can grow very big in time. Some of the data being stored there is sometimes not relevant anymore or not needed anymore.
You can regularly clean your log database table from old logs.
You can regularly clean your connections database tables from old connection entries.
You can regularly clean your guest database tables from old guest entries.
The configuration database table can grow as you install more and more modules and a lot of them do store data inside it. Some of them do remove this data when they are being uninstalled and other do not. This can result in a configuration database table quite big and that can be cleaned up.
Note that configuration table content is being loaded for every request PrestaShop handles so it can really slow your shop down.