This particular markdown takes a long time (21 seconds on my laptop) to parse:
https://github.com/date-fns/date-fns/blob/a9fc0c7b715883349555bfb94daa1059430eda52/src/locale/en-US/snapshot.md

$ time pygmentize -f terminal -l md -o /dev/null /tmp/snapshot.md

real    0m21.318s
user    0m21.306s
sys     0m0.012s

I've seen slow-ish parsing performance on markdown with tables before, however I'm not certain it's tables that's causing the issue.