Kilometertiden faller i takt med at løpsfarten øker.

Because Norwegian usually forms compound nouns as a single word.
So:

• kilometer
  • tid → kilometertid
• then the definite singular ending -en is added → kilometertiden

Literally, it is something like the kilometer-time, meaning the time per kilometer.

English often writes similar ideas as separate words, but Norwegian normally prefers compounds.

The ending -en means the in both words:

• kilometertiden = the kilometer time / the time per kilometer
• løpsfarten = the running speed

In Norwegian, the definite form is often used for general concepts that are treated like known quantities in a statement. Here, both are being discussed almost like variables: the per-kilometer time and the running speed.

English would often use no article or a different structure, but Norwegian commonly uses the definite form in this kind of general statement.

Faller literally means falls.

Norwegian often uses falle when a number, level, rate, price, or time goes down. So kilometertiden faller means the time per kilometer decreases.

That makes sense in a running context: if you run faster, the number of minutes needed for one kilometer becomes lower.

So even though English might prefer gets lower, decreases, or drops, Norwegian very naturally says faller.

I takt med at means something like:

• in step with
• as
• in proportion as
• along with the fact that

In this sentence, it introduces a relationship where one thing changes together with another:

• Kilometertiden faller i takt med at løpsfarten øker.
• The per-kilometer time decreases as the running speed increases.

The expression i takt med on its own is often followed by a noun phrase, while i takt med at is used before a full clause.

Because løpsfarten øker is a full clause, and at introduces that clause after i takt med.

Compare:

• i takt med løpsfarten = in step with the running speed
  
  • noun phrase
• i takt med at løpsfarten øker = as the running speed increases
  
  • full clause

So the at is there because the sentence does not just mention the running speed as a thing; it says something about it: that it increases.

Because at løpsfarten øker is a subordinate clause.

In main clauses, Norwegian usually has verb-second word order:

• Løpsfarten øker.
• Nå øker løpsfarten.

But after a subordinating conjunction like at, the clause has normal subject-verb order:

• at løpsfarten øker

So this is standard subordinate-clause word order.

Yes, basically. Løpsfart is a compound noun:

• løp = running / race
• fart = speed

So løpsfarten means the running speed.

Depending on context, it could mean the speed at which someone is running, rather than speed in a broader sense. It is more specific than just farten, which could refer to any kind of speed.

Yes. The present tense in Norwegian is often used for general truths, regular relationships, and statements of principle.

So here:

• faller = falls / decreases
• øker = increases

The sentence is not necessarily about one single moment only. It describes a general relationship: when running speed goes up, time per kilometer goes down.

This is very similar to how English uses the present tense in general statements.

Yes. A few alternatives are possible, for example:

• Jo høyere løpsfarten er, desto lavere blir kilometertiden.
• Kilometertiden synker når løpsfarten øker.

These all express roughly the same idea, but the original sentence sounds quite natural and slightly formal or explanatory.

The original wording is good for describing a measurable relationship, especially in training, sports, or technical contexts.

It is understandable and natural in contexts about running, training, and performance, but in everyday speech many people might also say things like:

• tid per kilometer
• pace
• tempo in some contexts

So kilometertid is not strange at all, but it belongs especially well in exercise or sports language, where compact compound nouns are very common in Norwegian.