Learn Polish: Delikatnie mówiąc, to był okropny film. - To put it mildly, it was an awful movie.

The word mówiąc is a present adverbial participle (an invariable word ending in -ąc). It translates literally to "speaking." In Polish, you pair these with adverbs to frame your attitude toward what you are saying, much like the English "-ing" forms in "frankly speaking" or "strictly speaking." Because it acts as a fixed introductory phrase, it never changes its ending to match the speaker.

What other adverbs are commonly paired with mówiąc to frame a sentence?

This is a very productive pattern for sharing opinions. You will frequently hear szczerze mówiąc (frankly speaking / to be honest), krótko mówiąc (in short / in a word), and ogólnie mówiąc (generally speaking). They all act as independent clauses that set the tone for the rest of your statement.

Could I use the perfective form and say delikatnie powiedziawszy instead?

Yes, but it sounds much more formal and slightly bookish. Powiedziawszy is a perfective participle meaning "having said." While phrases like szczerze powiedziawszy certainly exist and are understood, everyday spoken Polish strongly prefers the imperfective mówiąc for these attitude-framing expressions.

Why does the sentence use to był instead of to było, given that the pronoun to is neuter?