Long processing time when not using `--html-no-head-ids`

[Tachi107]

Hi! This is a minor issue. When not passing --html-no-head-ids, lowdown takes pretty long to process a big markdown file:

$ time lowdown ./@mui/x-internals/CHANGELOG.md > /dev/null 
real	0m0,579s
user	0m0,571s
sys	0m0,008s

$ time lowdown --html-no-head-ids ./@mui/x-internals/CHANGELOG.md > /dev/null 
real	0m0,030s
user	0m0,017s
sys	0m0,012s

$ time md2html ./@mui/x-internals/CHANGELOG.md > /dev/null 
real	0m0,022s
user	0m0,006s
sys	0m0,005s

I don't think that lowdown's performance is crucially important, but I just find it odd that a generating HTML head identifiers takes to much time compared to not doing it. As far as I can tell, no other option alters the performance this much.

The file I used to test this was https://github.com/mui/mui-x/raw/refs/tags/v8.14.0/CHANGELOG.md (nothing special about it; it was just a big file I had lying around on my pc).

Bye!