Keep in mind the A21. Is 5 years old. But here are some steps to speed it up a little.
Firstly go through the files and delete any you dont need. Or, this now being your phone you could try factory resetting it- that'll make it like it's "new"
There is an option to factory reset, which is different o just simply restarting it
Also. Try making sure low power mode isn't on.
Uninstall unused apps
Remove apps you don't use anymore to free up storage space and reduce background processes
Clear the cache
Clear the temporary storage area where your phone saves files to free up space and improve performance
Update to the latest software
Updating your phone to the latest version can improve performance, fix bugs, and provide security patches
Free up storage space
Delete old files, documents, and downloads you no longer need
Use the Device care feature
This feature provides an overview of your battery, storage, and RAM, and can automatically optimize your phone
Activate auto settings
Go to Settings, then Device care, then More options, then Advanced, and then activate the Auto settings