I tested this with my spawn script, but only at mapstart i'm afraid.
A soldier type that isn't on the map already (living or dead) will lag in a spawn script because he has to be loaded into memory.
A soldier that is placed on the map in the editor but is deleted at mapstart (put "deleteVehicle this" into his init line) ALSO lags. OpF is smart enought to see that the resources doesn't need to be loaded for this soldier. weird.
Dunno about units in midgame tho. Somebody should test this.