Learn Russian: Во-вторых, у нас мало времени.

Adverbs like во-вторых (secondly) and во-первых (firstly) are built using the prefix во- or в- plus the prepositional plural form of ordinal numbers. In Russian spelling, adverbs that start with в-/во- and end in -ых/-их must always be written with a hyphen.

Why is есть missing from the у нас (we have) construction?

In Russian, you omit есть when the sentence focuses on describing a trait or quantity of an object rather than simply stating that it exists or that you possess it. Since the main point here is the specific quantity of time you have (мало), есть is dropped.

Why does the word for time change to времени instead of staying as время?

Words that express quantity, like мало (little/few) or много (a lot), require the following noun to be in the genitive case. Время is one of a small group of special neuter nouns ending in -мя, and its genitive form is irregular: времени.

Is the comma after Во-вторых required?

Yes. Introductory words that organize the flow of speech, such as во-первых, во-вторых, or например, must always be set off from the rest of the sentence by a comma in Russian.