We found an interesting performance issue with, the specific example was with the grader interface in mod_assign but I believe it is a more generic bug with core_get_fragment

POST lib/ajax/service.php?sesskey=w44w4Y24QY&info=core_get_fragment

The proximate issue we had was that the course in question had a bunch of blocks which were somewhat expensive to render (block_progress in this case). By expensive I mean slow enough to have a measurable performance effect but not bad enough to be an issue in its own right.

But when the grading interface calls core_get_fragment then behind the scenes it is rendering a full page including all the blocks and then throwing it all away and just returning some subset which doesn't need it.