Learn Spanish (Spain): Nosotros somos rápidos y vosotros sois lentos.

Because they describe the pronouns nosotros (we) and vosotros (you all). Since these pronouns are masculine plural (which also covers a mixed group of males and females), the adjectives must match them by using the masculine plural ending -os.

If a group of only women were speaking, how would the first half of the sentence change?

It would become Nosotras somos rápidas. The pronoun changes to the feminine plural nosotras, and the adjective must match its gender and number by taking the feminine plural ending -as.

Why use somos and sois (from the verb ser) instead of estamos and estáis?

We use the verb ser (to be) for characteristics that are generally true or part of someone's identity. Being fast or slow is treated as an essential trait of the groups in this sentence, rather than a temporary condition.

What does vosotros sois mean, and is it used everywhere?

Vosotros is the informal plural 'you' (like 'you all' or 'you guys'), and sois is its matching form of the verb 'to be'. This specific pronoun and verb form are used almost exclusively in Spain. In Latin America, you would hear ustedes son instead.