Why is my new iPad slowing down and locking up because I don’t buy more iCloud?

If your iCloud storage is full and you use iCloud email, sync contacts, calendars, notes, photos or any data from your device to iCloud, then things will get flaky and you also risk losing data.

Turn those sync’d services off in iCloud settings to stop using them. If your storage is full and you use iCloud email, your email will be unable to send or receive new emails until you delete material from iCloud to make space available, or subscribe for more iCloud storage.

Check your iCloud storage:--> Check your iCloud storage online or on any device – Apple Support (UK)

If you need more space, but you don't want to buy extra storage, stop syncing data or creating backups to iCloud (use a computer instead).

Regards

Giulio