Avoid slow string byte counting functions

damus/Nostr/NostrEvent.swift

@@ -82,7 +82,7 @@ class NostrEvent: Codable, Identifiable, CustomStringConvertible, Equatable, Has
     }
     
     var too_big: Bool {
-        return self.content.underestimatedCount > 16000
+        return self.content.utf8.count > 16000
     }
     
     var should_show_event: Bool {