Speed-testing

We should do some serious performance profiling. While I don't have the evidence on hand, I would bet one of my kidneys that we have [massive N+1 problems](http://rails-bestpractices.com/posts/29-fix-n-1-queries). (Actually, now that I think back on it, I'm pretty sure I'm responsible for at least some of the N+1 code...) So, at some juncture, we should:

* [x] use [Rack Mini Profiler](https://github.com/MiniProfiler/rack-mini-profiler) and [YSlow](https://developer.yahoo.com/yslow/) to identify the source of slowness;
* [x] use [Bullet](https://github.com/flyerhzm/bullet) to reduce the number of N+1 queries;
* [ ] perform other optimizations that are, at this point, definitely not premature anymore.

Speed has, so far, been the chief complaint of almost every MT I talked to, so there's some case to make this top priority -- but it should probably wait for the next milestone, i.e. after we finalize the full testing coverage.