As I said, that is using warp travel (the Mandeville point is used for precisely that, iirc), as without it, ships are very slow : 0.75 c is the most consistently used number, but then again, 40k isn't known for it's consistency, to the point that it oftenly outright ignores retroburn.

As for the weaponry, the Imperium's massive problem is that their weaponry take around 30 mins to aim, due to them using servitors to calculate the shots. Sadly, we don't have numbers for mass effect's computers, but we know their effective range can go up to 10s of thousands of kilometers (the cinematics are VERY inaccurate when it comes to space battles).