ad a) heap consumption is too high for two reasons: firstly, the actual data I
care about is much less than there's data on the heap. Secondly, about half the
heap space is in LAG state. Here are profiles that will illustrate this:
http://imgur.com/wBWmJ&XN1mW&ltYNR.

- The first image shows 50% of the heap space being gobbled up with data that
shouldn't be there anymore (LAG)
- The second image shows the types that are in LAG state: ByteString and
HashMap. So, it seems I'm keeping around hash maps?

In Lag/Drag/Void/Use profiling, Lag is actually heap cells that are created too _early_. (Drag are those that are kept for longer than necessary.) Lots of Lag generally means your program is too strict - it is forcing structure long before it needs to. To fix it, you need to make things lazier. My first suspicion would fall on ByteString.