Not sure about recent systems - iPads/iPhones but on an OG iPhone SE 32GB I had just fall of last year, with under 500MB free space the iPhone was incredibly slow - and this is just using apps.
That said like
@BL4zD stated doesn't affect reading information.
Pulling up Contacts, existing saved pictures, alarms (already set to initiating the clock), calls and most SMS was not affected for performance. Listing to Music (locally stored not Apple Music) also not affected.
Social media apps constantly write to the storage via the streams of info being cached (written on device) - so those were immediately deleted by me.
Since the OS segments off part of the storage for itself, its' still shared storage.