"Heavy going" is a phrase used to describe something that is difficult or challenging to endure or complete. Synonyms for this phrase include "arduous," "onerous," "laborious," "strenuous," and "taxing." These words all convey a sense of struggle and effort required to overcome a difficult obstacle or task. Other synonyms for "heavy going" can also include "difficult," "tough," "challenging," "daunting," and "exhausting." These words are commonly used when describing a particularly demanding project, activity, or task that requires significant exertion, patience, and perseverance. Overall, "heavy going" is a versatile phrase that can be substituted with a variety of synonyms to effectively convey the same meaning.