My empire has spent the better part of a decade trying to figure out how to deal with size 8 ASMs that are moving at 30,000 km/s with multiple decoys.
After my explorer's first encounter ("Hey! There's a ship over there! It appears to be holding distance! Incoming Missiles! Full power to guns!" *Sound of five lasers missing* *SPLAT*) we've gradually been improving our designs.
In fact, you can see the progression in the lines of wrecks that are all spaced roughly five minute's travel time from each other!
For instance, we discovered that the tracking speed of your fire controls is really, really important. Turns out that just bringing those up from 2,000 km/s to 6,000 km/s tripled our to-hit% (who'd have guessed?). Now, on the fourth run into the hostile system, our latest fleet is our heaviest armored (going from 3 or 5 layers to 5/7), most well-armed, and most numerous. We're also trying out anti-missile jamming.
If this doesn't work, we're playing around with some turreted quad-lasers with the gearing needed so that they can track as quickly as the missiles move. We're avoiding that for the moment, because it will require an entirely new ship design - and probably a large one, at that, since both the turrets and the beam fire controls that they'll need are huge.
Normally I'd probably also go with trying to develop some AMMs and/or mass gauss defenses, but my empire has somehow literally never produced a single missile scientist, so I'm roleplaying that we're morally against such weapons ("dishonorable to strike at an enemy that's not at the space version of knife-fighting range.")
In other news, my starmap plot is a complete mess. For a good long while I was able to shift things around so that nothing overlapped. But then my explorers managed to find a link between the extreme far end of one of my lines and a system that's inside of two other loops. There's simply no way to rearrange the plot so that I don't have at least one system line crossing - it distresses me greatly (but not quite enough to SM out the offending connection.)