Unscientific observations so far:

1. Killing the phantom process once it's session is closed seems to help
2. Noticeable improvement seen after loading snapshots into phantom instead of doing the entire first replay there.

1. Setting the exported snapshot buffer to undefined should encourage the GC to remove it from memory later. A noticeable improvement occured.
2. A genuinely significant improvement would be to exchange the snapshots over socketio instead of the node-phantomjs library's bridge (which is seriously inefficient and a minefield of memory leaks).