سرعة التنزيل سريعة.

A fully vowelled version is:

سُرْعَةُ التَّنْزِيلِ سَرِيعَةٌ

Word by word:

• سُرْعَةُ = speed
• التَّنْزِيلِ = the downloading / the download
• سَرِيعَةٌ = fast

So the structure is literally:

• the speed of the download
  • is fast

A natural English translation is The download speed is fast.

Because Arabic usually leaves out the verb to be in the present tense.

So in English you say:

• The download speed is fast

But in Arabic, for the present tense, you normally just say:

• سرعة التنزيل سريعة

This is called a nominal sentence.

If you wanted a past or future meaning, then Arabic would use an actual verb, for example:

• كانت سرعة التنزيل سريعة = The download speed was fast

This is a nominal sentence made of:

• مبتدأ (topic/subject): سرعةُ التنزيلِ
• خبر (predicate): سريعةٌ

So:

• سرعةُ التنزيلِ = the download speed
• سريعةٌ = fast

Together: The download speed is fast.

Yes. سرعة التنزيل is one noun phrase, and it is an iḍāfa construction.

An iḍāfa is a structure like:

• X of Y

Here:

• سرعة = speed
• التنزيل = the download / downloading

So:

• سرعة التنزيل = the speed of the download or download speed

This is very common in Arabic.

Because in an iḍāfa construction, the first noun usually does not take الـ.

So Arabic says:

• سرعةُ التنزيلِ

not:

• السرعةُ التنزيلِ

The definiteness comes from the second noun:

• التنزيل is definite because it has الـ
• therefore the whole phrase سرعة التنزيل is definite

So even without الـ on سرعة, the whole phrase can still mean the download speed.

Because it is the second part of an iḍāfa.

In an iḍāfa:

• the first noun is called مضاف
• the second noun is called مضاف إليه

The second noun, مضاف إليه, is put in the genitive case.

So in:

• سُرْعَةُ التَّنْزِيلِ

the ending on التنزيلِ is genitive.

A learner-friendly breakdown is:

• سُرْعَةُ = nominative, because it is the subject of the sentence
• التَّنْزِيلِ = genitive, because it is the second term in the iḍāfa
• سَرِيعَةٌ = nominative, because it is the predicate

Because سريعة agrees with سرعة, and سرعة is grammatically feminine.

In Arabic, adjectives and predicate adjectives usually agree with the noun they describe in gender and number.

So:

• سرعة = feminine noun
• therefore the predicate is سريعة = feminine form of fast

If the noun were masculine, you would use سريع instead.

Yes. They come from the same root:

• س ر ع

This root has to do with speed or quickness.

So:

• سرعة = speed
• سريعة = fast

Arabic often builds related words from the same root, so it is very normal to see this kind of resemblance.

Because in a basic sentence like this, the predicate is often indefinite.

So Arabic says:

• سرعة التنزيل سريعة

not usually:

• سرعة التنزيل السريعة

Here سريعة is not acting like an adjective inside the noun phrase. It is the predicate of the sentence: it tells you something about سرعة التنزيل.

Compare the difference:

• سرعة التنزيل سريعة = The download speed is fast
• سرعة التنزيل السريعة = the fast download speed

The second one is just a noun phrase, not a complete sentence.

It is written:

• التنزيل

But because ت is a sun letter, the ل of الـ is assimilated in pronunciation.

So it is pronounced roughly:

• at-tanzīl

not:

• al-tanzīl

That is why the fully vowelled form is:

• التَّنْزِيل

with a shadda on ت.

So the whole sentence is pronounced approximately:

• surʿatu t-tanzīli sarīʿatun

In careful reading or formal grammar, the full form is:

• سُرْعَةُ التَّنْزِيلِ سَرِيعَةٌ

But in normal pause at the end of the sentence, final case endings are often not pronounced fully.

So you will often hear something closer to:

• سُرْعَةُ التَّنْزِيلِ سَرِيعَة

And in less formal spoken Arabic, case endings are often dropped even more broadly.

So for learning purposes, it is good to know both:

• formal grammatical form: سريعةٌ
• common paused pronunciation: سريعة