The “monthly lag” is high on the priority list. That is all I can say for now, sorry!
As I´m sure you already know, in Train Fever you cannot build roads crossings over railroad tracks. This also applies to the cities. When the city wants to expand its road network, and has nowhere to go – because for example a railroad tack goes all around (or around most parts of) the city – this can cause additional lags.
You can “help” the city by providing bridges, tunnels or crossings over your tracks to make it easier to expand.
Best,
Tom