Today we had 40k requests like this:
Almost all of them are unique urls (31k urls) with different ids for each assignment activity and I think possibly for each individual assignment. As far as I can tell these 40k urls map to 4 unique stamps: smile, sad, tick, cross.
1) These all map to the same urls so they get cached
2) All 'stamp' files are served as public cache control immutable so varnish / cdn's can serve these instead of hitting php land