Die aflaaispoed is vinniger as die oplaaispoed.

Die is the Afrikaans definite article, so here it means the.

It appears twice because there are two separate noun phrases:

• die aflaaispoed = the download speed
• die oplaaispoed = the upload speed

A useful thing to remember is that Afrikaans uses die for all genders, and also for plural nouns, so it does not change the way English articles sometimes do.

Afrikaans very often forms compound nouns by writing them as a single word.

So:

• aflaai
  • spoed → aflaaispoed
• oplaai
  • spoed → oplaaispoed

English usually writes these as two words, like download speed and upload speed, but Afrikaans normally joins them.

They are built from parts that can help you remember them:

• af = down / off
• op = up
• laai = load

So:

• aflaai is literally something like load down → download
• oplaai is literally something like load up → upload

That makes aflaaispoed and oplaaispoed very logical compounds.

Vinnig means fast.
To make the comparative faster, Afrikaans usually adds -er:

• vinnig = fast
• vinniger = faster

So vinniger as means faster than.

This is similar to English adjectives like fast → faster.

In comparisons, as means than.

So:

• vinniger as = faster than
• groter as = bigger than
• kleiner as = smaller than

This can feel strange to English speakers because English as is a different word, but in Afrikaans as is the normal word after a comparative.

A useful comparison:

• vinniger as = faster than
• so vinnig soos = as fast as

Because the adjective comes after is, where Afrikaans normally uses the adjective without the extra -e ending.

Here it is in a predicative position:

• Die aflaaispoed is vinniger.

But when an adjective comes before a noun, it often does take -e:

• die vinnige verbinding = the fast connection

So after is, vinniger is the expected form.

Yes, in this sentence it is very similar.

The structure is:

• Die aflaaispoed = subject
• is = verb
• vinniger = complement
• as die oplaaispoed = comparison phrase

So the pattern is essentially:

The download speed + is + faster + than the upload speed

That said, Afrikaans still follows its own verb-placement rules. In a normal main clause like this one, the finite verb comes early, usually in second position.

No. Afrikaans does not capitalize all nouns.

So aflaaispoed and oplaaispoed stay lowercase.
The only reason Die has a capital letter here is that it is the first word of the sentence.

That is different from German, where all nouns would be capitalized.