This started when emacs would freeze every time I opened a file that had some ascii art text in the comment. I realized that by removing the comment the issue cleared up. I then went through all removed one character type at a time. Turns out that underscores are the culprit here.
To repro: hold down the "_" key. after a few have printed you should see things slow down, and then halt. For me, it is difficult to get past 15 or so on my high end macbook pro.
Commenting this line eliminates the problem
(modify-syntax-entry ?_ "w" coffee-mode-syntax-table)