Simple.
A tad slower than the shockwave from deathStars reactor.
How do we define that?
Distance from core to surface is known from the blueprints.
Now we know the distance from surface to core and the time explosion took to make it.
Now we know: v = s/t
so velocity of the shockwave is = distance / time.
Tiefighters difference can be calculated from the shockwaves total time - shockwave reaches Tiefighter time.
The problem is that Star Wars does not actually take place in a setting where we can compare it to any form of objective reality. Things just happen with no actual logic applied at all- Han getting to Bespin with no hyperdrive, Star Destroyers rumbling in the noiselessness of space, and fighter craft tearing around a vacuum, banking to make turns as if they were in atmosphere.
Speeds are exactly one such thing. The idea of a speed limit in space makes very little sense until you are at ridiculous extremes. Yet most modern day jet fighters go faster than the sublight speeds given for an X-Wing.
So when you try and go in-depth into analysing it, all you really end up with is a load of gumf and queeq and myself found the extremes to which some people were preoared to argue about such a completely arbitrary concept rather amusing.
You have to abandon any pretence that this can be analysed and rationalised. it's just a load of dumb fun.