I had the same '13.75GB used 13.75GB' issue after the Uitakka update which made the phone barely usable when running more than 4 apps. Deleting mail caused a 'not responding' message every time I tried to delete a mail.

They said in their release notes that the phone might lag in particular low memory instances but it was pretty awful, so I investigated why.

They're going to have to fix the btrfs issues as I imagine a lot of non-technical users have made their phones much, much worse.