Yeah, I was also confused by using a project that's been unmaintained for several years as the pinnacle of Markdown performance, but I have never once thought about the performance of Markdown parsers (I can imagine someone on the scale of reddit or github would care about performance of the MD parser, but I never have for my projects) so I don't know enough to argue about whether performance has improved since then.
At SunSed we provide a cloud IDE to our users so they can easily update their website. Every time they make a change the IDE automatically saves the file and on the server we had to compile Markdown -> HTML+SunSed(custom HTML tags) -> Executable. Not to mention bulk website upload, backups and restores (a website could have thousands of pages). Markdown was the slowest part with the most resource usage.
I actually bench marked all of the Markdown implementatinos back in 2016 (or early 2017). https://github.com/vmg/sundown was by far the fastest!