Improve master proxy performance with large numbers of generations

We have observed that the proxy performs significantly worse when a large number of recovery generations are outstanding. We should investigate the source of this performance degradation.

Start by reproducing or profiling the master proxy with a large number of outstanding recovery generations, as described in the issue. Trace where performance degrades and document the bottleneck; done means identifying the source of the degradation and a concrete direction for improving it.

Written by the indexing model from the issue text.