Me sorprendió lo rápido que llegó el taxi, aunque había mucho tráfico.

Me is an indirect object pronoun meaning to me.

So me sorprendió literally means it surprised me or it was surprising to me.

In this sentence, what surprised the speaker is the whole idea in the next part:

lo rápido que llegó el taxi

So the structure is basically:

• Me sorprendió X = X surprised me

Here, X is how quickly the taxi arrived.

Sorprendió is the preterite, which is used for a completed reaction or event.

The speaker is talking about a specific moment of surprise:

• Me sorprendió... = It surprised me...

If you said me sorprendía, it would sound more like an ongoing or repeated feeling in the past, which does not fit as naturally here.

So:

• sorprendió = a completed reaction at a particular moment
• sorprendía = used more for ongoing, habitual, or background surprise

Lo rápido que means something like how fast or how quickly.

This is a very common Spanish structure:

• lo + adjective/adverb + que

It is used to express the degree of something.

Examples:

• lo difícil que fue = how difficult it was
• lo caro que es = how expensive it is
• lo rápido que llegó = how quickly it arrived

Here, rápido is functioning adverbially, describing how the taxi arrived.

So:

• Me sorprendió lo rápido que llegó el taxi
  = I was surprised by how quickly the taxi arrived

Here, lo is a neuter article/pronoun. It does not mean it or the in the usual way.

It turns an adjective or adverb into an abstract idea:

• lo bueno = what is good / the good thing
• lo importante = what matters / the important thing
• lo rápido = how fast / the speed of it

So lo rápido que llegó el taxi is not about a masculine noun.
It is an abstract expression meaning the extent of the taxi’s quickness, or more naturally in English, how quickly the taxi arrived.

In Spanish, adjectives like rápido, lento, claro, etc. are often used as adverbs, especially in everyday language.

So both of these are possible:

• llegó rápido
• llegó rápidamente

But llegó rápido is much more natural and common in normal speech.

That is why lo rápido que llegó el taxi sounds very natural.

Llegó is preterite because it refers to a single completed action:

• the taxi arrived

The arrival happened once and finished.

If you used llegaba, it would suggest an ongoing action, repeated action, or background description, which is not what the sentence is doing.

So:

• llegó = it arrived
• llegaba = it was arriving / used to arrive / would arrive in a descriptive context

Here the sentence is about the completed event of the taxi arriving.

Había is the imperfect, and it describes background circumstances.

The traffic is the situation that existed while the taxi was arriving:

• aunque había mucho tráfico = although there was a lot of traffic

This is classic imperfect use: setting the scene or describing conditions in the past.

If you said hubo mucho tráfico, it would sound more like there was a lot of traffic as a completed event or occurrence, rather than background context.

So in this sentence:

• llegó = main completed event
• había = background condition

Aunque can be followed by either indicative or subjunctive, depending on meaning.

Here it takes the indicative because the speaker presents había mucho tráfico as a real, known fact:

• aunque había mucho tráfico = although there was a lot of traffic

If the information were uncertain, hypothetical, or not confirmed, Spanish could use the subjunctive.

So:

• aunque había mucho tráfico = the traffic was definitely there
• aunque hubiera mucho tráfico = even if there were a lot of traffic / whether or not there was a lot of traffic

In your sentence, the speaker clearly means it as a real fact, so indicative is correct.

Because tráfico here is being used as an uncountable/general noun, like traffic in English.

So:

• mucho tráfico = a lot of traffic

You do not normally say el mucho tráfico here.

Compare:

• Había mucho tráfico = There was a lot of traffic
• Había mucho ruido = There was a lot of noise
• Había mucha gente = There were a lot of people

This is a very common pattern with hay / había / hubo, etc.

Both are possible in Spanish.

• llegó el taxi
• el taxi llegó

Spanish has more flexible word order than English. After a verb like llegar, it is very common for the subject to come after the verb, especially when the subject is being introduced or mentioned as part of the event.

Here, llegó el taxi sounds completely natural.

It helps keep the focus on the action and on lo rápido que llegó.

In standard Spanish, for this meaning, lo rápido que is the normal structure.

So:

• Me sorprendió lo rápido que llegó el taxi = correct and natural

Using qué rápido here is not the standard choice in this kind of sentence.

Qué rápido llegó el taxi by itself can work as an exclamation:

• ¡Qué rápido llegó el taxi! = How quickly the taxi arrived!

But after me sorprendió, Spanish normally prefers:

• Me sorprendió lo rápido que llegó el taxi

Yes, absolutely. That is another very natural way to say something similar.

Compare:

• Me sorprendió lo rápido que llegó el taxi
• Me sorprendió que el taxi llegara tan rápido

The difference is mostly one of structure and emphasis:

1. Me sorprendió lo rápido que llegó el taxi
   Focuses on the degree of speed: how quickly

2. Me sorprendió que el taxi llegara tan rápido
   Focuses more on the fact that the taxi arrived so quickly

Also notice the grammar:

• After me sorprendió que..., Spanish uses the subjunctive: llegara

Both are correct and natural, but the original sentence highlights the speed more strongly.

The comma helps separate the main idea from the contrasting clause:

• Me sorprendió lo rápido que llegó el taxi
• aunque había mucho tráfico

This is similar to English punctuation before although in many cases.

It makes the contrast clearer:

• the taxi arrived very quickly
• this is surprising because there was a lot of traffic

In short, the comma is natural and helps readability.

It behaves like a noun-like expression inside the sentence, even though it contains a clause.

The whole chunk:

• lo rápido que llegó el taxi

functions as the thing that surprised the speaker.

That is why it can be the subject-like content after me sorprendió.

You can think of it as meaning:

• the fact/how quickly the taxi arrived

So grammatically, it is more than just a simple noun phrase, but in the sentence it acts as a single unit: the thing that caused surprise.