1. AFAIK it's the most popular Markdown gem around (ref. https://www.ruby-toolbox.com/categories/markup_processors) so using it in a project is more "default" than it is "optimization".

2. With over 10M downloads, I assume at least some of them spend large percentages of time using it.

3. It's Markdown; replicating the idiosyncrasies of SmartyPants (https://daringfireball.net/projects/smartypants/) in a different language is not something many people are going to want to do.

I'm not saying that people should or should not use Redcarpet; just that it's unlikely that anyone engaged in "premature optimization" here.