Learn Spanish (Latin America): ¿Por qué el servicio es lentísimo?

It comes from the adjective lento (slow). To make it mean "extremely slow" or "super slow," we drop the final -o and add the suffix -ísimo. Because it describes el servicio, which is a masculine singular noun, it keeps the masculine singular ending -o.

Why does lentísimo have a written accent mark?

When you add the -ísimo or -ísima suffix to any adjective, the spoken stress always falls on the í. In Spanish spelling rules, any word where the spoken emphasis is on the third-to-last syllable (len-tí-si-mo) automatically requires a written accent mark.

Why use es here? If the service is just slow today, shouldn't it be está?

You can actually use either, but they give off different vibes. Using es (from the verb ser) suggests that the service being extremely slow is a general characteristic of this place—it is always this slow. If you used está (from the verb estar), you would be pointing out that the service is unusually slow right now.

Why is Por qué written as two words with an accent mark?