If someone studies assiduously, does it mean they're working really hard or really slowly?

Assiduous study could be fast, slow, or moderately paced. In any case, working assiduously means the student won't give up, even if a more interesting pastime comes along. Assiduousness calls for sticking to a plan, and carrying out the effort with constant, focused attention.

In Henrik Ibsen's play An Enemy of the People, "People's Messenger" publication editor Hovstad says,

And, in the matter before us, it is now an undoubted fact that Dr. Stockmann has public opinion against him. Now, what is an editor's first and most obvious duty, gentlemen? Is it not to work in harmony with his readers? Has he not received a sort of tacit mandate to work persistently and assiduously for the welfare of those whose opinions he represents? Or is it possible I am mistaken in that?