Eu prefiro o novo, porque o antigo é lento.

Yes. In Brazilian Portuguese, subject pronouns are often optional because the verb form already shows who is doing the action.

So both of these are natural:

• Eu prefiro o novo, porque o antigo é lento.
• Prefiro o novo, porque o antigo é lento.

Using eu can add emphasis, contrast, or clarity. For example, Eu prefiro... can sound a bit like I prefer..., especially if you are contrasting your opinion with someone else’s.

Because preferir is conjugated irregularly in the first person singular.

The infinitive is preferir. Its eu form is:

• eu prefiro

Some useful forms are:

• eu prefiro
• você prefere
• ele/ela prefere
• nós preferimos
• vocês preferem
• eles/elas preferem

So prefiro is simply the correct present-tense form for I prefer.

Here, novo and antigo are adjectives being used like nouns. This is very common in Portuguese.

• o novo = the new one
• o antigo = the old one

The noun is understood from context and left out. For example, the hidden noun could be carro, celular, modelo, computador, and so on.

So the sentence works like:

• Eu prefiro o novo, porque o antigo é lento.
• I prefer the new one, because the old one is slow.

Because the implied noun is masculine singular.

For example, if the hidden noun were:

• o carro → o novo / o antigo
• o celular → o novo / o antigo

If the implied noun were feminine, you would use:

• a nova / a antiga

And in the plural:

• os novos / os antigos
• as novas / as antigas

So the article and adjective must match the gender and number of the noun being understood.

Both can relate to old, but they are not always used in the same way.

• antigo often means former, previous, older, or from before
• velho often means old in the sense of aged, worn out, or not new

In this sentence, o antigo sounds very natural for the old one / the previous one, especially when contrasting it with o novo.

For example:

• o modelo antigo = the older/previous model
• o modelo velho = the old model, possibly emphasizing that it is worn out or outdated

So antigo is a very good choice here.

Because porque means because and introduces a reason.

In this sentence:

• Eu prefiro o novo, porque o antigo é lento.
• I prefer the new one, because the old one is slow.

This porque answers an unspoken why?

Portuguese has several similar-looking forms:

• porque = because
• por que = why / for which reason
• por quê = why at the end of a question
• porquê = the reason (a noun)

Here, only porque fits.

The comma separates the main statement from the clause giving the reason.

• Eu prefiro o novo, porque o antigo é lento.

In everyday writing, many people also write this without the comma:

• Eu prefiro o novo porque o antigo é lento.

Both are common. The comma can make the sentence easier to read, especially when the second part adds an explanation.

Because ser and estar are used differently.

• é lento suggests a more general or characteristic quality
• está lento suggests a temporary condition

So:

• o antigo é lento = the old one is slow by nature / as a general characteristic
• o antigo está lento = the old one is slow right now, perhaps temporarily

In this sentence, the speaker is probably describing a general problem with the old one, so é lento makes sense.

Because preferir already contains the idea of liking one thing more than another.

So Portuguese usually does not need an extra mais.

Natural patterns include:

• Prefiro o novo ao antigo.
• Prefiro o novo em vez do antigo.
• Eu prefiro o novo, porque o antigo é lento.

A common learner mistake is something like:

• prefiro mais o novo do que o antigo

That is usually unnecessary. Preferir already expresses preference by itself.

Yes. A very common direct version would be:

• Eu prefiro o novo ao antigo.

That means I prefer the new one to the old one.

Your original sentence adds the reason:

• Eu prefiro o novo, porque o antigo é lento.

So the original is not just stating a preference; it is also explaining it.

It depends on what thing you are talking about. For example:

• Eu prefiro o carro novo, porque o carro antigo é lento.
• Eu prefiro o celular novo, porque o celular antigo é lento.
• Eu prefiro o modelo novo, porque o modelo antigo é lento.

In real Portuguese, repeating the noun twice can sound a bit heavy, so speakers often shorten it to:

• Eu prefiro o novo, porque o antigo é lento.

That is one reason this structure is so common.