@Ljevid
Have you ever had a UCM Massdriver bs PHR Broadside duel? That should be more sluggish, for Shaltari you can use Lances (except the Battleship) to a similar effect. Try if you like that, honestly do so.
Personally I doubt that level of weapon effectiveness works in a game where you usually don´t have that many turns to fire. A lot of scenarios have (thanks to approach types) some sort of reserve that arrives later, usually there is 0 shooting T1 and often in T2 as well, I even had games where T3 passed with mostly cautious approaching in silent running by both sides.
So in general T3-4 have the main action if you wanted the combat to be more "sluggish" you would have to extend this period in game or risk a game where losses matter little or mostly to late to influence half of the scoring.
In regards to realism, I agree that the combat deployment feels odd if it happens every game. However otherwise you would essentially remove the ground aspect if you relegate it to only moving troop carriers in if you secured the void zone. That is a sacrifice of sci-fi "realism" for game purposes. The whole planet focus would feel odd and it would be a "standard" space combat game.
For me this is the same "lack of sci-if realism" as (at least in theory) balanced fleets, rarely have truly balanced fleets or armies fought each other, yet in tabletop this happens all the time, or at least is usually attempted all the time.
I believe the deadliness is an integral part of the game and intentionally built into it.
Partially however I doubt that the full extend was realised. In particular as the differences in damage between certain weapon systems have not really been fixed, in some cases not even attempted to do so if we look at Massdrivers vs BTL. The Shaltari Lances are also in a sad state with the only concession was the Jade change to 2+ after several threads that showed the maths between them and the Topaz, which was immediately undermined by creating a point difference between the Jade and Topaz after those weapon systems were properly in balance for the first time.
So no I do not think the designers fully understand the maths behind their weapon systems. Thus fail to see their deadliness in comparison to each other. Which is quite common in tabletop gaming systems.