Sia che la trama ti sembri lenta, sia che ti sembri avvincente, voglio sapere cosa ne pensi.

Sia che ... sia che ... is a fixed structure meaning whether ... or ....

So:

• Sia che la trama ti sembri lenta
• sia che ti sembri avvincente

means:

• Whether the plot seems slow to you
• or whether it seems gripping to you

Italian often repeats sia che before both alternatives, unlike English, where we usually just say whether ... or ....

Here sia is the subjunctive present of essere.

In this expression, though, you do not need to think of it as the normal meaning is. In sia che ... sia che ..., it functions as part of a grammatical pattern introducing alternatives.

This structure normally triggers the subjunctive in the following clause because it presents possibilities or alternatives rather than straightforward facts.

Sembri is the present subjunctive of sembrare.

That happens because sia che ... sia che ... is followed by the subjunctive:

• che la trama ti sembri lenta
• che ti sembri avvincente

If this were a plain statement, you might expect the indicative:

• La trama ti sembra lenta. = The plot seems slow to you.

But after sia che, Italian uses the subjunctive:

• Sia che la trama ti sembri lenta...

Ti means to you.

In Italian, sembrare often works like to seem plus an indirect object:

• Mi sembra interessante. = It seems interesting to me.
• Ti sembra noioso? = Does it seem boring to you?

So:

• la trama ti sembri lenta = the plot seems slow to you
• ti sembri avvincente = it seems gripping to you

The ti does not mean you as the subject. It means to you.

Because in the second half it is understood and does not need to be repeated.

The full idea is:

• Sia che la trama ti sembri lenta, sia che la trama ti sembri avvincente...

But Italian, like English, often avoids repetition when the meaning is clear. So the second la trama is omitted:

• sia che ti sembri avvincente

This is very natural.

Voglio sapere means I want to know.

It introduces the main clause of the sentence. Everything before it sets up the alternatives:

• Whether the plot seems slow to you
• or whether it seems gripping to you

Then the speaker says:

• voglio sapere cosa ne pensi = I want to know what you think about it

So the structure is:

• [Whether X or Y], I want to know Z.

Ne here means about it.

The expression is:

• pensare di qualcosa = to think about something
• pensarne = to think about it / to have an opinion about it

So:

• Cosa pensi della trama? = What do you think about the plot?
• Cosa ne pensi? = What do you think about it?

In your sentence, ne refers to the matter being discussed, most naturally the plot or possibly the work in general, depending on context.

Pensi is the present subjunctive of pensare, second person singular.

It is used because it follows voglio sapere cosa..., which often takes the subjunctive when the speaker is referring to someone’s opinion, thought, or attitude in a subordinate clause.

So:

• voglio sapere cosa ne pensi = I want to know what you think about it

Italian commonly uses the subjunctive after verbs of wanting, doubting, feeling, or asking, especially in subordinate clauses.

Yes.

Both are correct:

• cosa ne pensi
• che cosa ne pensi

They both mean what do you think about it?

Cosa is a bit shorter and very common in both speech and writing.
Che cosa is slightly more explicit, but not very different in meaning.

Yes. Both adjectives agree with la trama, which is feminine singular.

• la trama = feminine singular
• lenta = feminine singular form of lento
• avvincente = an adjective whose form is the same for masculine/feminine singular

So:

• la trama ... lenta
• la trama ... avvincente

Even though avvincente does not change visibly here, it still agrees with la trama.

Yes. It is perfectly normal.

The understood subject is la trama, and the rest is:

• ti = to you
• sembri = seem
• avvincente = gripping

So the full form would be:

• la trama ti sembri avvincente

Since la trama was already mentioned, Italian leaves it out the second time:

• ti sembri avvincente

This is a very natural kind of omission.

It is mostly neutral.

Nothing in it is especially slangy or especially formal. However:

• ti shows the speaker is addressing you informally
• with Lei, it would change, for example:
  • Sia che la trama Le sembri lenta, sia che Le sembri avvincente, voglio sapere cosa ne pensa.

So the original sentence is appropriate for normal conversation with someone you address as tu.

Yes. A simpler, more everyday alternative might be:

• Che la trama ti sembri lenta o avvincente, voglio sapere cosa ne pensi.
• Mi interessa sapere cosa ne pensi, sia che la trama ti sembri lenta sia che ti sembri avvincente.

But the original sentence is completely natural and elegant. The sia che ... sia che ... structure is just a more explicit and polished way to say whether ... or ....