The reason wasn't JS performance or Electron, but simply that bracket colorization was bolted on in an extension which had to make use of the existing extension

The reason wasn't JS performance or Electron, but simply that bracket colorization was bolted on in an extension which had to make use of the existing extension API, which obviously didn't expose the needed internal features to properly implement bracket colorization. The exact same thing would have happened in a native editor if its plugin interface didn't anticipate a particular requirement.