I checked the FS1 techroom and there is nothing on Subspace. The FS2 techroom does have information on subspace, but nothing suggesting that intrasystem is faster than intersystem.
As for the mission in question, it is: SM2-05 "The Sicilian Defense"
Event: Alba Incoming
Command says: Pilots, the NTC Alba has run our blockade of the Capella jump node. We are tracking the vessel to your rallying point. The Alba will emerge from subspace momentarily.
Alba's Arrival:
-is event true delay
----Alba Incoming
----10
So from this mission alone, we know that "A" the Alba jumps, and Command tracks it through subspace. And that the Alba takes at least 10 seconds to arrive, more like minimum 20 as it takes time for Command to read the report and then open his mouth to tell everyone about it.
Therefore from this one cannon mission, intrasystem jumps are clearly not instantaneous.