Yes, and a similar question is the organization of the thing being acted on by the passes. If I understand correctly, this is in scheme and the things being acted on are trees with pointers. A performance optimized compiler, on the other hand, will probably use some sort of array-based implementation of trees.

There's also a question of data about the trees (like, a flow graph) being recomputed for each nanopass. Also expensive.

Nanopass uses structures internally to represent the programs.

The Nanopass dsl just gives the user a nicer syntax to specify the transformations.