A feltöltési sebesség lassú.

Yes. A is the Hungarian definite article, so here it means the.

A few useful points:

• A is used before a consonant sound.
• Az is used before a vowel sound.
• Hungarian often uses the definite article in places where English might or might not use the, especially when talking about something specific or already understood from context.

So A feltöltési sebesség means the upload speed.

In Hungarian, the verb to be is usually omitted in the present tense, third person, when the predicate is a noun or an adjective.

So:

• A feltöltési sebesség lassú. = The upload speed is slow.

Literally, it looks like:

• the upload-speed slow

This is completely normal Hungarian.

But compare:

• Én lassú vagyok. = I am slow.
• Te lassú vagy. = You are slow.

Here the verb appears because it is not third person.

Feltöltési means related to uploading or upload in the sense of upload speed.

It comes from:

• feltölt / feltölteni = to upload
• feltöltés = uploading, upload
• feltöltési = uploading-related, upload-

So:

• feltöltési sebesség = upload speed

This -i ending is very common in Hungarian for making an adjective that means related to X.

For example:

• iskola = school

• iskolai = school-related, school

• nyár = summer
• nyári = summer, summertime

So feltöltési is not a separate basic dictionary word you memorize by itself first; it is a very regular formation.

Because feltöltési is functioning like an adjective modifying the noun sebesség.

So the structure is:

• feltöltési = upload / uploading-related
• sebesség = speed

Together:

• feltöltési sebesség = upload speed

This is similar to English noun phrases like:

• internet speed
• download speed
• upload speed

Hungarian often uses an adjective-like form before the noun in this kind of expression.

Because in this sentence lassú is not directly modifying the noun inside the noun phrase. It is the predicate adjective: it tells you what the upload speed is like.

So:

• A feltöltési sebesség lassú. = The upload speed is slow.

But if you want to say the slow upload speed, then lassú goes before the noun:

• a lassú feltöltési sebesség = the slow upload speed

This is an important difference:

• A feltöltési sebesség lassú. = The upload speed is slow.
• A lassú feltöltési sebesség... = The slow upload speed...

Not in the way adjectives do in many Indo-European languages.

Hungarian adjectives do not agree for gender, because Hungarian has no grammatical gender. They also do not change in singular attributive use.

Here:

• sebesség = singular
• lassú = singular form, unchanged

So:

• a lassú sebesség = the slow speed

In predicate position, plural can matter:

• A sebességek lassúak. = The speeds are slow.

Notice that the adjective can take a plural ending in that kind of sentence.

But in the singular sentence you gave, lassú stays just lassú.

It is in the nominative, the basic dictionary form.

The sentence has:

• A feltöltési sebesség = the upload speed → subject
• lassú = slow → predicate adjective

There is no special case ending here because sebesség is simply the subject of the sentence.

A rough pronunciation guide is:

• A ≈ short aw
• feltöltési ≈ fel-turl-tay-shee (very approximate)
• sebesség ≈ sheh-beh-shayg
• lassú ≈ law-soo

A few pronunciation notes:

• s in Hungarian sounds like English sh
• sz sounds like English s
• e is usually like e in bed, but shorter/cleaner
• é is a longer ay-like sound
• ú is a long oo
• ö and ő are rounded front vowels, which English does not really have

Stress in Hungarian normally falls on the first syllable of each word:

• A FEL-töl-té-si SE-bes-ség LAS-sú

Yes, it is a very normal and common technical expression for upload speed.

You will often see it alongside:

• letöltési sebesség = download speed

So these are a very useful pair to learn:

• feltöltési sebesség = upload speed
• letöltési sebesség = download speed

This kind of formation is very common in technical Hungarian.

Yes, absolutely.

Both are correct:

• A feltöltési sebesség lassú.
• Lassú a feltöltési sebesség.

The second version puts more emphasis on slow. It can sound a bit more natural in conversation, depending on context.

Very roughly:

• A feltöltési sebesség lassú. = neutral statement
• Lassú a feltöltési sebesség. = It’s the upload speed that’s slow / The upload speed is slow

Hungarian word order is flexible, and changing the order often changes emphasis rather than basic meaning.

It is natural Hungarian, especially in technical contexts, customer support, internet settings, diagnostics, or reviews.

Depending on context, people might also say:

• Lassú a feltöltés. = The uploading is slow.
• Lassú a feltöltési sebességem. = My upload speed is slow.
• Nagyon lassú a feltöltési sebesség. = The upload speed is very slow.

So your sentence is perfectly normal, but in everyday speech people sometimes choose a shorter version if the context is obvious.

This is a very useful distinction.

• feltöltés = uploading, upload
  This is a noun.

• feltöltési = upload-, related to uploading
  This is an adjective form used before another noun.

Compare:

• A feltöltés lassú. = The uploading is slow.
• A feltöltési sebesség lassú. = The upload speed is slow.

So feltöltés can stand on its own, but feltöltési usually introduces another noun, such as:

• feltöltési sebesség = upload speed
• feltöltési idő = upload time

It can mean both.

Sebesség is the general word for speed. Depending on context, it can refer to:

• physical speed
• typing speed
• processing speed
• internet speed
• upload/download speed

So in this sentence, because of feltöltési, it clearly means network/upload speed, not movement through space.