tpope/vim-markdown — pull request #8 by thedjinn (2011-11-13 13:18:50+00:00)
Title: fix performance issue with detecting setext-style headers

The current setext-style header rules are O(n^2) which makes the entire markdown highlighting very slow.

Because the current rules do not work for nested setext headers anyway (some parsers do support this though) we can make it significantly faster by only checking the match at the start of the line.