Colin O'Dell has a post to his site talking about how he used the Blackfire.io service to optimize league/commonmark, a library that's a part of The PHP League making it easier to work with Markdown content in PHP.

As you may know, I am the author and maintainer of the PHP League's CommonMark Markdown parser. [...] This last goal [of being well-written and super-extensible so that others can add their own functionality] is perhaps the most challenging, especially from a performance perspective. Other popular Markdown parsers are built using single classes with massive regex functions.

He shares some brief benchmarks of parsing the same document with four different parsers, including an older version of CommomMark (coming in last, unfortunately). With the goal of optimization in mind, he tried out the Blackfire.io service to locate the bottlenecks in his library. Based on the results from the tests he found two optimizations that could be made relatively easily: switching from single-character parsing to regex and more correctly replacing newlines with breaks where needed.