IPad Pro 11 / 13 ProMax
Proton Mail has a new feature that can auto-forward gmail and handle it for you. Works great except it runs really slow and frequently freezes the device. Affects both the iPad and iPhone. Runs a little better on Safari but still laggy.
The Proton account runs fine on S23 Ultra, ThinkPhone, MBP, ThinkPad. Size fo the Gmail account was 4700 documents, Contacts, and Calendar.
Reported via Feedback
Reported to Proton.
Guess this is a case of welcome to 16.5 Pita 4.
Update: seeing postings in Reddit for the same issue for iOS.