They're probably reducing the brickcount to free up space for more bricks, not reduce lag.
They're at 200k bricks and not done with map. You can't blame them for wanting bricks to be used as well as possible. Also, past 400k or so game gets very difficult to work with. Any large large scale builder will tell you that.
But it's not the amount of bricks it's the amount of ghosts. You can have a stuffton of simple 1x1 bricks in a giant cube because the game renders them without interior faces. If you place 400k worth of door bricks that are separated from eachother the game crashes. I mean I guess it's nice to have a lower brickcount but it's not what decides whether the game crashes or not.