I don't know the difference between startup and precompilation, and I do not really care, but if I launch a julia script from the command line it is unbearably slow for no apparent reason. Octave, on the other hand, launches instantly and starts making computations.

This is understandable, because julia is not intended to be used that way. You are supposed to "live" inside the repl. However, I prefer tools that are flexible enough that can be used comfortably in non-intended ways.