Well roughly how long did it take the Lucifer to traverse the coridoor at the end of FS? (assuming you dont blow it up, that should be the canon travel time. Gameplay reasons omitted) There's your baseline to calibrate other subspace travel times by.
We never see anything relating to how long time passed between the time you enter the node in the second-to-last mission and the briefing in the last mission. For all we know, it could have been a day or so that passed and you're just finally starting to catch up with the Lucifer.
Another possibility is from a gameplay perspective, no one would want to play a mission that takes the same amount of time that it may (or may not) take in fluff. i.e., if subspace travel between the systems took days, or even weeks, no one would play a mission that long.
There's also the chance, however unlikely, that Sol and Delta Serpentis are really, really close together.