These are the antonyms of the word: “efficient” that came up in google search:

inefficient disorganized

disorganized inadequate

unsystematic ineffective

confusing disordered

disorderly disjointed

muddled irregular

cumbersome sloppy

careless unprofessional

frivolous impractical

flippant thoughtless

amateur unbusinesslike

unserious unimportant

trivial