I've noticed a hard performance hit when trying to highlight a comment in banner-style:

//////////////////////////////////////
// EXTRA FANCY COMMENT NOBODY NEEDS //
//////////////////////////////////////
console.log("fancy");

Highlighting this snippet takes around 5 Seconds on my machine

Somebody posted a similar comment to this in our Mattermost Chat (which uses highlight.js) and rendered the channel virtually unusable due to the load on the clients.

I've built a Codepen which isolates the problem: Codepen