Yes that's correct. I believe each object loads into memory on a layout but once you leave that layout it is unloaded from memory, therefore you only need to load the relevant objects for that layout at any time.
Loading all the game assets in your game on an initial layout won't work as they won't be there on the new layout. When you leave the layout you don't have to worry about any other objects being left behind unloaded, it is done automatically I think.
The fact that there is a noticeable FPS drop on an object being created but not for future instances of that object seems like a problem with the object.
You could try: leaving the enemy object on the layout view; when it is preloaded on moving to the layout during the game, does the FPS spike occur? Or is it just when the enemy is created/spawned?