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The six ATX heading regions (markdownH1..markdownH6) and the ordered
list marker start with a bounded whitespace count (" \{,3}#" and
"\%(\t\| \{0,4}\)\<\d\+\."), which gives the regexp engine no first
character to anchor on. The NFA engine therefore re-scans these at every
line start; the backtracking engine rejects non-matching lines quickly.
Force the backtracking engine with \%#=1 on these patterns. Highlighting
is unchanged. On a mixed 6000-line document this cuts total :syntime
engine work roughly in half (each heading region ~-97%, ordered marker
~-44%).
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The six ATX heading regions (markdownH1..markdownH6) and the ordered list marker start with a bounded whitespace count (" {,3}#" and "%(\t| {0,4})<\d+."), which gives the regexp engine no first character to anchor on. The NFA engine therefore re-scans these at every line start; the backtracking engine rejects non-matching lines quickly.
Force the backtracking engine with %#=1 on these patterns. Highlighting is unchanged. On a mixed 6000-line document this cuts total :syntime engine work roughly in half (each heading region ~-97%, ordered marker ~-44%).