When AI are spawned after mission start using the createUnit Array or any BIS function, the server load increases significantly more that an equivalent number of AI introduced into a mission via the editor.
This can be demonstrated using the attached example missions that use 400 AI on each mission on opposing sides (the effect definitely exists with far fewer AI, but 400 helps to illustrate it).
When I run the tests on my dedicated server, the editor placed AI give 32 - 48 Server FPS when AI starts and 22 FPS when they get moving, while the spawned AI give 7-9 FPS when they are all spawned and 11 FPS when they get moving. {F21025}