As others wrote, Pandoc is Haskell so it compiles to a fairly efficient binary.

But more importantly, unlike the various Markdown flavors or AsciiDoc, it is incredibly extensible thanks to the combination of custom filters and the possibility to add HTML classes and attributes. One can write filters to leverage the class/attribute information and perform transformations at the AST level, which basically lets you define a DSL with an arbitrary number of custom elements.

I wrote a collection of filters for the publication of a large online legal playbook. Not only did Pandoc make it possible to introduce different kind of custom elements that don't exist in plain Markdown or AsciiDoc, but by using different filters it was possible to use a single Markdown source to generate both the book and various summaries such as a list of examples, a list of civil code clauses etc. I don't know Haskell that well so I used Rust for the filters, but that worked very well.

Pandoc is IMO a very underrated tool.