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It's called on the main thread. So if it takes any time (e.g. compressing some data), your UI will lock up. Worse, it locks up your UI when auto-save kicks in (i.e. not in response to a user action). Same for ReferenceFileDocument.fileWrapper Documentation says background thread: Ensure that types that conform to this protocol are thread-safe. In particular, SwiftUI calls the protocol’s methods on a background thread. Don’t use that thread to perform user interface updates. Use it only to serialize and deserialize the document data. Stack trace: * thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 3.1 * frame #0: 0x000000010e0cf634 AsyncSave`AsyncSaveDocument.fileWrapper(snapshot="Hello, world!\n\nTest auto save\n\n", configuration=$s7SwiftUI30FileDocumentWriteConfigurationVD @ 0x00007ff7b1e307d0, self=(text = "Hello, world!\n\nTest auto save\n\n")) at AsyncSaveDocument.swift:51:47 frame #1: 0x000000010e0cf9ca AsyncSave`protocol witness for ReferenceFileDocument.fileWrapper(snapshot:configuration:) in conformance AsyncSaveDocument at <compiler-generated>:0 frame #2: 0x00007ff84a271109 SwiftUI`___lldb_unnamed_symbol129907 + 146 Must be a bug eh? With the newer concurrency stuff, I would think that your FileDocument would have to be Sendable. I tried making it Sendable in sheer desperation, but fileWrapper was still called on the main thread.