`NSTextStorage` vs. `TextStorageSwiftSubclass`
Calling `-[NSTextStorage attributesAtIndex:effectiveRange:]` is nearly 3 times as slow for `TextStorageSwiftSubclass` compared to `NSTextStorage`. Time profiling shows that this is caused by the bridging of the Objective-C dictionary return value to Swift. Effectively, this means we cannot use Swift if we want to subclass `NSTextStorage`.
Sample project and time profiler trace are attached.