MIRVs affect all of your opponent's territory, so it makes zero sense that the time it takes for a MIRV to hit is affected whatsoever by the position of the silo or where you click. It seems like players controlling territory at the north of the map are typically advantaged by this, since the projectile travels to the north of the map to launch.
It absolutely needs to be a flat timer of 5 seconds (10 seconds is frankly too long for the opponent to react) that is the same regardless of map location