Arduous
Etymology
From Latin "arduus" (steep, high, difficult). The physical image of a steep climb sits underneath the abstract sense: an arduous task is one you toil up, gaining ground slowly against gravity.
adjective · /AR-joo-us/
Demanding great and sustained effort; hard and tiring.
What it names
Difficulty of the uphill, effortful kind — steep work that taxes stamina, not just patience.
Example
The translation was arduous — six months of work for a book you could read in an afternoon.
Related idea
Arduous stresses the strenuous exertion required; tedious stresses the boredom of monotony, so a task can be arduous without being dull, or tedious without being hard.