I'm finding the "jTab and ABCjs Formatting Sandbox" is taking up to 20 seconds to render fully, and it seems to be because of this post, which is testing a particularly long block of ABC.
I'm on an old, low spec office PC running a bunch of applications at once, so not exactly giving poor ABCjs a chance. Nonetheless, if the amount of ABC code on a page is unlimited, then the potential is there to grind any browser to a halt, whether inadvertently or maliciously.
*UPDATE - I copied Dom's ABC from the sandbox post, made it three times as long and posted it as an answer here. The page took 2 minutes to render. I doubt this will generally be an issue as people are most likely to use ABC for posting short musical snippet - more likely to be a problem if someone decides to be a nuisance. Maybe another way to go would be restricting access to this functionality to users with a certain amount of rep?