I literally switched on "Enable offline support", caching "All mail" offline on my iPhone a few months ago. Tons of free space, only using 4GB for offline.
But when my phone is actually offline (on a plane or elevator) it beachballs when trying to find something.
(I work for Fastmail). That sounds odd – have you contacted our support team so we can look into it? https://www.fastmail.com/support/