There is a little lag while the game spawns in creatures, but by not having them constantly there, a server does not have to constantly keep track of NPCs and their states/scripts/etc. Depending on the size of a server (if going multiplayer), it's likely a better idea to go with spawning rather than permanent placement.
Personally, I only place certain NPCs that will need to be the same and in the same places at most times (such as merchants). Village commoners, monsters, etc. I spawn in via scripting or encounter triggers. I then have the module remove such creatures from any particular area a little while after all PCs leave that space using a "area cleaner" script (which also removes stray items from the ground, gets rid of remains/loot bags left behind, and re-closes doors that started out that way).