Question about command line's delay and array

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So, I use command to load robots/ground by this line:

ign service -s /world/free_world/create --reqtype ignition.msgs.EntityFactory --reptype ignition.msgs.Boolean --timeout 300 --req 'sdf_filename: ''"models/sdf/new_ground.sdf" pose: {position: {x: 0, y: 0, z: 0}} ''name: "ground_%s" ''allow_renaming: true'

SO if I ran this with a loop, each line takes one second to load approximately. So, it seems that it has a delay. Is there any way to reduce the delay?

Also, is there a way to load more than 5 models at once?

Like for example: ign service -s /world/free_world/create --reqtype ignition.msgs.EntityFactory --reptype ignition.msgs.Boolean --timeout 300 --req 'sdf_filename: ''"model1.sdf, model2.sdf,model3.sdf" 'pose: {position: {x: 0, y: 0, z: 0},{x: 2, y: 0, z: 0},{x: 4, y: 0, z: 0}} '

Obviously, above is just guess or psuedo command line. Is there any official command for it?