Individual student progress reports taking long to load

[Aypak]

I'm running ka-lite 0.16.2 on ubuntu 14.04. This version has greatly reduced the statrup time, and time taken to load exercises, which is very much appreciated. However, i noticed that when loading individual student progress reports it takes about 2 minutes to load. This is observed both when logged in as a student and a coach. There are no javascript errors in the console. It seems like it could possibly be a performance issue.

[benjaoming]

Tried to figure out something with django-debug-toolbar, but need more work since the page itself seems to be fine - most likely a performance would be in the API endpoints.

Adding a 0.18 milestone to ensure that we keep tracking this as a bug, even though it might not be fixed for 0.17 series.