Mióta lassú a letöltési sebesség?

Mióta asks about the starting point of a situation that is still continuing now.

So here it means:

• Since when is the download speed slow?
• or, in more natural English,
• How long has the download speed been slow?

Hungarian often uses mióta where English may choose either since when or how long depending on what sounds more natural.

Because Hungarian normally leaves out the verb to be in the present tense, third person in simple sentences like this.

So:

• A letöltési sebesség lassú. = The download speed is slow.

There is no van here, even though English needs is.

That is completely normal Hungarian. In fact, adding van here would usually sound wrong.

Letöltési means something like download-related or for downloading.

It comes from:

• letöltés = download
• -i = an adjective-forming ending

So:

• letöltés = the act of downloading / a download
• letöltési = download-, downloading-, related to download

That gives:

• letöltési sebesség = download speed

A useful parallel is:

• feltöltési sebesség = upload speed

Both are possible, but lassú a letöltési sebesség is very natural Hungarian.

In sentences with an adjective predicate and no expressed is, Hungarian often puts the predicate first:

• Lassú a letöltési sebesség. = The download speed is slow.

If you say:

• A letöltési sebesség lassú

that is also grammatical, but it can sound a bit more like As for the download speed, it is slow.

So the version in your sentence is a very normal, idiomatic word order.

Because Hungarian uses the definite article here: a = the.

So:

• a letöltési sebesség = the download speed

Hungarian often uses the definite article in places where English might also use the, especially when talking about a specific thing in the situation.

Also, Hungarian has two forms of the definite article:

• a before consonants
• az before vowels

Since letöltési starts with l, the correct form is a.

It can correspond to either one in English.

The core idea of mióta is:

• from what point up to now?

So depending on context, English may translate it as:

• Since when is the download speed slow?
• How long has the download speed been slow?

Hungarian is not making a strong distinction here in the same way English often does.

Common answers include a starting point or a duration.

Examples with a starting point:

• Tegnap óta. = Since yesterday.
• Reggel óta. = Since this morning.
• Múlt hét óta. = Since last week.

Examples with a duration:

• Két napja. = For two days.
• Egy órája. = For an hour.

You can also answer with a full sentence:

• Tegnap óta lassú a letöltési sebesség.
• Két napja lassú.

Yes. Very much so.

• óta means since / for in time expressions
• mióta means since when?

Examples:

• tegnap óta = since yesterday
• két órája = for two hours
• Mióta lassú? = Since when has it been slow?

So when you hear mióta, it is useful to connect it mentally with time expressions like tegnap óta, reggel óta, and so on.

These two question words look a little similar, but they ask completely different things:

• Mióta = since when / how long
• Miért = why

So:

• Mióta lassú a letöltési sebesség? = asking about time
• Miért lassú a letöltési sebesség? = asking about reason

That is a very common beginner confusion, so it is worth learning them separately.

Yes, you could.

As a statement:

• A letöltési sebesség lassú. = The download speed is slow.

As a yes-no question, with question intonation:

• A letöltési sebesség lassú? = Is the download speed slow?

But that is a different kind of question from the original sentence.

The original:

• Mióta lassú a letöltési sebesség?

is specifically asking about when the slowness started.

Yes. It is the normal technical expression.

A related phrase is:

• letöltés sebessége = literally the speed of the download / of downloading

That version is more of a possessive-style phrase, while letöltési sebesség is the standard compound term, like a label or category.

So if you are talking about internet performance in general, letöltési sebesség is the most natural choice.