You have a point.
Because modern computers are fast and have plenty of memory.
Not sure about that. I notice something in my big scripts (like Favmenu witch is 5000+ lines). When tray icon is loaded after everything else, there is a 1 second delay on my 2x3GHz CPU with 1GB memory. When I set it to be first line, I never see H icon.
I understand your concern, but I think that even loading and parsing a 1MB library file
I think you overestimated complexity of this. AHK still need to parse each function it includes. Currently, if it encounteres missing funciton it will report. SO, the only thing that has to be done is not to report it, but to search in the rest of the included files. This can be cached, so it will fast.
Plus I don't see any advantage, unless parsing and referencing is done one time before shipping AutoHotkey and stored in some "database". If this is done dynamically, not only AutoHotkey will still have to load the library and parse it (at least to skip non-relevant functions) anyway, but has to resolve dependencies!
Crhis might be able to provoide some insight for this situation.