Unexpected Performance Boost Using Subscripts Over Explicit Casting

I have a function that returns a specific nested dictionary. It looks like this:

private func fontDict() -> [String: AnyObject]? {
  if let fonts = configDict["Fonts"] as? [String: AnyObject] {
    return fonts[self.rawValue] as? [String: AnyObject]
  }

  return nil
}

My view performance was lagging so I did a quick profile. This method was clocking in at 156ms on my main thread (it’s called a lot). Not good.

Refactoring to this:

private func fontDict() -> [String: AnyObject]? {
  return configDict["Fonts"]?[self.rawValue] as? [String: AnyObject]
}

And the time drops to 36ms. Quite the performance boost!