Performance issue in groupBy

I have an array of 300 non-trivial objects, which I want to filter with `groupBy`. When I attempt to do that, my whole applications slows to a crawl.

This is due to enormous amounts of time being spent in `getHashKey`, which `JSON.stringify`s the whole array to use that as the key for `$$cache`.

If there's an alternative approach, I'd love to hear it :)