I did implement an A* routing "offline" routing algorithm based on GTFS data + walking paths (OptiTravel) and built a GTFS server (to easily serve the data, do geospatial queries, ...) a few years ago. Granted that this was a university project, for some cities the calculation was rather intensive (e.g: London). I might be wrong though.
Cool projects! I actually had a look at you gtfs-server, to use Rust for processing GTFS files. Processing GTFS (basically large zipped text files) is quite resource intensive indeed, especially if you want to do it for the whole planet. What would you do differently now?