Would it be possible to slow down fleet speeds on the system map?
Simple answer, yes, it's possible to do right now via code in a mod.
Question then becomes, as Reshy put it: what's the point?
Pure travel is the least-interesting aspect of any RPG, and in a space RPG where we don't have anything to do other than dodge enemy fleets, it's doubly boring. If there were random encounters, arcade challenges, etc., to make it challenging and interesting (all of this is possible atm or very nearly) then, well, that half-hour spent going from Gamma Spiculae IV to Circe A is time spent on meaningful, fun activity, not just a waste of our precious time on this Earth.
If anything, I honestly feel that this aspect of Starfarer is one of the biggest in need of a re-think or elaboration as the game matures.
I've played enough RPGs that there is very little joy in travel for its own sake, especially in a space RPG on a large scale (i.e., there aren't varied environments that matter).
This isn't like Fallout 3, where I was like, "whoa, there's a new rock I haven't seen, next to a building- wait, are those Deathclaaaaaaaaah.... <thud, squish>".
Instead, it's like, "hey, it's a planet with art I've seen before, with a station that looks like every other station, circling a sun that looks like a sun". Or, if playing Vanilla, it's like, "hey, it's Corvus, that place that hasn't changed a bit". But I only play in Corvus if testing somebody else's mod.
I've even set up the randomizer in Vacuum to produce some major random variations... it just doesn't matter a whole lot. It's still star, planet, moon, asteroid belt, dodge-the-fleets. Nowhere feels unique or interesting.
Freelancer had a pretty good fix for this problem, by making the travel faster and getting rid of a lot of the boring stuff ++ having a lot of support for interesting content to make the individual systems memorable- things like different ambient backgrounds, gas clouds that made it hard to see around your ship and reduced sensor distances, radioactive danger zones, even minefields and automated enemies hiding out. This plus fast-travel made for a beautiful and memorable but non-dragging travel experience.
But even that got fairly dull and frustrating after a while. If you went on one of the really epic high-end trade runs, it could take an hour or more, just traveling. All we're doing during this is moving stuff on a FedEx to make a little money, yo; it was way, way more fun to take missions instead, even if it didn't pay as well.
I really think Fallout 3 got it right, by allowing us to skip this stuff once we've been somewhere before.
I just want to hit a button to send me Here and either have random encounters that are fun and interesting or just Get There. No in-between with dull dodge-the-fleets stuff.
There are so, so many other ways to handle chases between fleets, for example; have our ship's "mass sensors" set to different "sensitivities", which would directly impact how many random encounter choices we get during a journey, for example.