Your observation that assignment rather than using substitution is exactly on track! Great job!

As far as performance goes, my experience is that split is an "expensive thing" CPU wise. A regex that tests whether a line starts with X is a relatively inexpensive thing. I have also found that the join() operation is very performant - i.e. joining 12 things together is way faster than splitting one thing into 12 things. Anyway I think the performance enhancement here is to not do a split at all unless you have to - my code is shown below which uses a regex to decide whether to split or not split.