This isn't strictly necessary for this task, but having seen the code, we should look at rewriting it. It's currently adding one event handler per grade cell, and then when the tooltip is opened, we add more (which are never removed).
We also add large numbers of event handlers for other areas.

Temporarily removing these does seem to improve the performance of the page, but it's difficult to tell subjectively, and it will likely make more of a difference on tablets.