I’ve spent the last couple days optimizing commonmark.js, which is now just a little slower than marked.js:
commonmark.js markdown->html x 709 ops/sec ±1.28% (95 runs sampled)
showdown.js markdown->html x 248 ops/sec ±1.90% (87 runs sampled)
marked.js markdown->html x 729 ops/sec ±2.20% (94 runs sampled)
markdown-it markdown->html x 986 ops/sec ±1.15% (94 runs sampled)
Note that the benchmarks are highly dependent on the specific input used; I used a 10 MB Markdown text composed of twenty concatenated copies of the first edition of Pro Git. (make benchjs will run the benchmark above; make benchjs BENCHINP=foo.txt will use foo.txt for the source.)