O aplicativo do banco fica mais lento quando o sinal da internet está fraco.

Fica comes from ficar, which often means to become or to get in this kind of sentence.

So fica mais lento means gets slower or becomes slower.

If you used é mais lento, it would sound more like a permanent characteristic: is slower. But here the app changes condition depending on the internet signal, so fica is more natural.

• O aplicativo fica lento = The app gets slow / becomes slow
• O aplicativo é lento = The app is slow

Both can be possible in Portuguese, but they are used a little differently.

• lento is an adjective: slow
• devagar is usually an adverb: slowly

In this sentence, mais lento describes the app itself, so an adjective makes sense:

• O aplicativo fica mais lento = The app gets slower

If you said funciona mais devagar, then devagar would describe how it works:

• O aplicativo funciona mais devagar = The app works more slowly

So mais lento focuses on the app’s condition, while mais devagar would focus more on the manner of operation.

Do is a contraction of de + o.

• de = of / from
• o = the
• de + o = do

So:

• o aplicativo do banco = the bank’s app / the app of the bank

This structure is very common in Portuguese:

• o carro do meu pai = my father’s car
• a porta do quarto = the bedroom door

Portuguese often uses de where English might use apostrophe ’s.

Da is a contraction of de + a.

• de = of / from
• a = the
• de + a = da

So:

• o sinal da internet = the internet signal

This is the same pattern as do banco, but now the noun internet is treated as feminine, so it uses a, which contracts to da.

Está is used for a temporary state or condition, while é is used more for identity, definition, or more permanent characteristics.

Here, the internet signal is not permanently weak; it is weak at that moment or in that situation. So está fraco is the natural choice.

• O sinal está fraco = The signal is weak right now / at the moment
• O sinal é fraco = The signal is weak by nature or in general

In everyday Brazilian Portuguese, signal strength is normally expressed with estar:

• O Wi-Fi está ruim
• A conexão está fraca

Here sinal means signal, specifically the strength or quality of the internet connection.

So o sinal da internet está fraco means the internet signal is weak.

In Brazilian Portuguese, sinal can also mean other things depending on context, such as:

• traffic light
• sign
• indication
• phone signal / reception

But in this sentence, it clearly refers to network signal or connection strength.

Portuguese uses definite articles much more often than English.

In English, you might say:

• Bank app
• Internet signal

But Portuguese usually prefers:

• o aplicativo do banco
• o sinal da internet

This is very normal and natural. Portuguese tends to specify nouns with articles even where English would leave them out.

So even though the sentence may feel article-heavy to an English speaker, it sounds completely standard in Portuguese.

Yes, very often in Brazil people do say app in everyday speech.

So these are both possible:

• O aplicativo do banco fica mais lento...
• O app do banco fica mais lento...

Aplicativo is a bit more formal or neutral, while app is very common in casual speech and writing.

Because both adjectives agree with masculine nouns:

• aplicativo is masculine, so: mais lento
• sinal is masculine, so: fraco

If the nouns were feminine, the adjectives would change:

• a conexão está fraca
• a página fica mais lenta

Portuguese adjectives usually agree in gender and number with the noun they describe.

Yes, in this sentence quando means when.

It introduces the condition or situation in which the app becomes slower:

• quando o sinal da internet está fraco = when the internet signal is weak

In many cases, quando works just like English when:

• Quando chego em casa, tomo banho.
• Quando chove, fico em casa.

Here it is not asking a question; it is connecting two parts of the sentence.

Sometimes yes, but the meaning shifts a bit.

• quando = when
• se = if

So:

• O aplicativo do banco fica mais lento quando o sinal da internet está fraco. = The app gets slower when the internet signal is weak.

This suggests that whenever that situation happens, the app gets slower.

• O aplicativo do banco fica mais lento se o sinal da internet estiver fraco. = The app gets slower if the internet signal is weak.

This sounds more conditional and hypothetical. Also, with se, Portuguese often prefers estiver rather than está in this kind of sentence.

So quando fits well if the speaker is describing a regular pattern.

This is the normal Portuguese word order:

subject + verb + complement

• O aplicativo do banco = subject
• fica = verb
• mais lento = complement

So:

• O aplicativo do banco fica mais lento = The bank app gets slower

Portuguese is fairly flexible, but this order is the most natural and neutral here. Moving things around is possible in some contexts, but it might sound marked or unnatural to a learner.

Yes. Ficar is a very common verb with several meanings, including:

• to stay
• to become / to get
• to be located
• to keep / remain in some contexts

Here, the structure ficar + adjective usually means to become/get:

• ficar lento = to get slow
• ficar triste = to get sad
• ficar pronto = to become ready

So in this sentence, fica mais lento clearly means gets slower, not stays slower.

The original sentence already sounds natural. But in everyday speech, Brazilians might also say:

• O app do banco fica mais lento quando a internet está ruim.
• O aplicativo do banco fica lento quando o sinal tá fraco.
• O app do banco trava mais quando o sinal da internet tá fraco.

A few notes:

• app is very common
• tá is the spoken short form of está
• internet está ruim is very common in casual speech
• trava means freezes / lags / glitches, depending on context

So the original sentence is correct and natural, but real-life speech may use shorter or more informal wording.