Some preliminary testing show that select-keys can be almost twice as fast if it were rewritten to use a transient map rather than a proper map as it does today.
In order to do so, select-keys would need to move to after where transient and its friends are defined.
If there is any interest in pursuing this, I'd be happy to provide a problem statement, more data on the performance, a couple of possible solutions, and perhaps ultimately a patch.
