It's not only about a Spawn Room outside the Map, you can build a Spawn Room somewhere Inside the Map just as well, which is closed from the Outside by a Door or a corridor / floor. That means you can't see any Monsters or Projectiles as you Login so it will already help to prevent this Problem.
As far as I know not, only if you see the Monsters / Projectiles will cause this Problem. For example, in Maps where a Door or Floor separating the Spawn Room from the Action, I never noticed any Login Problems so far.