Ideas for further performance improvements

[mandolyte]

Just so we don't lose track of ideas we might try to improve performance. I found that prefetching all the org/lang repos actually decreased performance slightly. Not enough to undo the prefetching, but enough to wonder what might be going on to slow things down.

• Use a different library for zip reading that performs better (do not know if any are optimized for reading, but this seems to be a slow part of the app)
• Process all repos for a given book concurrently; for example using PromiseAll. Currently repos are processed in series.
• Ensure that the new unzipped-store is always checked first (since it is new, perhaps something was missed?)

[mandolyte]

@PhotoNomad0 's work has optimized performance for current library and this is sufficient.