Cooldoings Aais:
"Apple Pencil lagging only in Notability: [...]for some reason when it comes to Notability the pencil is lagging and this is hindering all my work of note preparations.[...]"
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Troubleshooting a Lagging App:
Clear Space:
Try clearing internal storage, though use of the following:
What Space to Clear:
A. Clear Temporary Internet Files:
Clear your Temporary Internet Files(Cookies & Cache) of Safari:
And...
B. Clean TV and Music Libraries:
Remove all unneeded files from your TV and Music Libraries. Full-Movies take up 2-4GB typically, and a bunch of those would take up a load of storage, ideally. So, if you have any movies that are not needed, then delete them.
And...
C. "Other" being the Culprit:
Use this Link: How to Check the Storage on your iPhone, iPad, and iPod touch - Apple Support. The storage named "Other" is the grey bar. So, to clear that, this is what to perform: Clear Other storage on your iPhone or iPad - Apple Support