I think this is present so that if log messages are serialized to a central location it is possible to distinguish which process/slave they came from.
That functionality isn't currently being used, and there is an overhead from initialising the field. So it could be removed, or the initialisation could be commented out.
It would be instructive to trace the work involved in logging a message to check there aren't any other unnecessary bottlenecks - e.g. cloning strings etc. Maybe it would be better to create the message, and then format the string directly (but cleanly).