Antonyms of adj effortless

2 senses of effortless

Sense 1
effortless (vs. effortful) -- (requiring or apparently requiring no effort; "the swallows glided in an effortless way through the busy air")

effortful (vs. effortless) -- (requiring great physical effort)
        => arduous, backbreaking, grueling, gruelling, hard, heavy, laborious, operose, punishing, toilsome -- (characterized by effort to the point of exhaustion; especially physical effort; "worked their arduous way up the mining valley"; "a grueling campaign"; "hard labor"; "heavy work"; "heavy going"; "spent many laborious hours on the project"; "set a punishing pace")
        => dragging -- (marked by a painfully slow and effortful manner; "it was a strange dragging approach"; "years of dragging war")
        => exhausting, tiring, wearing, wearying -- (producing exhaustion; "an exhausting march"; "the visit was especially wearing")
        => heavy, labored, laboured -- (requiring or showing effort; "heavy breathing"; "the subject made for labored reading")
        => labor-intensive, labour-intensive -- (requiring a large expenditure of labor but not much capital; "cottage industries are labor intensive")
        => leaden, plodding -- ((of movement) slow and laborious; "leaden steps")
        => Sisyphean -- (both extremely effortful and futile)
        => arduous, straining, strenuous -- (taxing to the utmost; testing powers of endurance; "his final, straining burst of speed"; "a strenuous task"; "your willingness after these six arduous days to remain here"- F.D.Roosevelt)

Sense 2
casual, effortless -- (not showing effort or strain; "a difficult feat performed with casual mastery"; "careless grace")

INDIRECT (VIA easy) -> difficult, hard -- (not easy; requiring great physical or mental effort to accomplish or comprehend or endure; "a difficult task"; "nesting places on the cliffs are difficult of access"; "difficult times"; "why is it so hard for you to keep a secret?")

Similarity of adj effortless

2 senses of effortless

Sense 1
effortless (vs. effortful) -- (requiring or apparently requiring no effort; "the swallows glided in an effortless way through the busy air")
       => facile -- (performing adroitly and without effort; "a facile hand")
       => unforced, unstrained -- (not resulting from undue effort; not forced; "a voice with a pleasingly unforced quality"; "his playing is facile and unstrained")
          Also See-> easy#1

Sense 2
casual, effortless -- (not showing effort or strain; "a difficult feat performed with casual mastery"; "careless grace")
       => easy (vs. difficult) -- (posing no difficulty; requiring little effort; "an easy job"; "an easy problem"; "an easy victory"; "the house is easy to heat"; "satisfied with easy answers"; "took the easy way out of his dilemma")