Learn Spanish (Latin America): Le reportan la lentitud al operador. - They report the slowness to the operator.

In Spanish, you can use the third-person plural (the "they" form, like reportan) to talk about an action where the person doing it is unspecified or doesn't matter. It is a common way to avoid the passive voice. It translates to the general "they" in English ("they report the slowness") or to a passive phrase ("the slowness is reported").

Why do we need the pronoun le at the beginning if we already say al operador at the end?

This is called indirect object duplication. Even though al operador explicitly tells us who is receiving the report, Spanish generally requires the indirect object pronoun le to point to that person anyway. Leaving it out here would sound unnatural.

Could I use the passive se here instead to avoid mentioning a specific subject?

Yes, you could say Se le reporta la lentitud al operador. Both are natural ways to express an unspecified subject. The impersonal reportan sounds a bit like a casual "They report the slowness," while se reporta sounds slightly more formal, like "The slowness is reported."

How does the word lentitud relate to the word for slow?

It is the noun form of the adjective lento (slow). By replacing the final vowel with the suffix -itud, you create lentitud (slowness). You will spot this same pattern in other Spanish nouns, which often correspond to English words ending in "-ness" or "-tude".