> ...as well as increasing the Node.js heap size to 32Gb.

> ...also saw that the process’s heap size stayed fairly constant at around 1.2 Gb.

This is because 1.2 GB is the max allowed heap size in v8. Increasing beyond this value has no effect.

> ...It’s unclear why Express.js chose not to use a constant time data structure like a map to store its handlers.

It it is non-trivial (not possible?) to do this in O(1) for routes that use matching / wildcards, etc. This optimization would only be possible for simple routes.