It's slightly more efficient to append single characters to ``++StringBuffer++`` and ``++StringBuilder++`` instances as ``++char++``s, than as ``++String++``s. That is, it's more efficient to put a single ``++char++`` in single quotes, rather than double quotes.
I really doubt this one should be activated by default. This is really something that you should activate either, to be anal on your style or because you have a perf critical application that actually cares about this, so any way I would not include it in default profile.