kanozyo ha sigoto ga isogasikute mo, messeezi no hensin ga hayai desu.

They are doing different jobs.

• 彼女は sets the topic: as for her
• 仕事が marks the subject inside the clause 仕事が忙しくても
• 返信が marks the subject of 早い

So the structure is roughly:

• 彼女は = speaking about her
• 仕事が忙しくても = even if work is busy
• メッセージの返信が早いです = her message replies are quick

A very common pattern in Japanese is:

• Topic + は
• then inside the sentence, smaller parts still use が for their own subjects

So having both is completely normal.

忙しくても means even if busy or although busy.

It is built from the い-adjective 忙しい:

1. 忙しい
2. change it to the て-form: 忙しくて
3. add も
4. → 忙しくても

So:

• 忙しくて by itself often means and busy
• 忙しくても means even if busy / although busy

This 〜ても pattern is very common:

• 高くても = even if it is expensive
• 雨でも = even if it is raining
• 行っても = even if someone goes / even if I go

In this sentence, 仕事が忙しくても means even if she is busy with work.

Japanese often says 仕事が忙しい to mean busy with work.

Literally, it looks like work is busy, but the real idea is:

• someone’s work schedule is busy
• or someone is busy because of work

So 仕事が忙しい is a very natural Japanese way to talk about being busy due to work.

If you said 彼女が忙しい, that would simply mean she is busy.
If you say 彼女は仕事が忙しい, it gives a more specific reason: as for her, work is busy.

メッセージの返信 means reply to messages or message reply.

The particle の connects two nouns. It can show many kinds of relationships, not just possession. Here it means something like:

• 返信 related to messages
• reply of messages
• more naturally in English, reply to messages

So:

• メッセージ = message
• 返信 = reply, response
• メッセージの返信 = message reply / replying to messages

This is a very common noun-linking pattern in Japanese.

Because Japanese often describes the speed or timing of the reply itself, rather than using an adverb first.

So 返信が早い literally looks like:

• the reply is early/quick

But in natural English it usually means:

• she replies quickly
• she is quick to reply

This is a common Japanese way of expressing habits or characteristics:

• 足が速い = someone runs fast / has fast legs
• 決断が早い = quick to make decisions
• 返信が早い = quick to reply

So the focus is on the reply as a characteristic.

Both can be read はやい, but they are used differently.

• 早い = early, soon, prompt
• 速い = fast in speed

For replying to messages, the important idea is how soon the reply comes, not physical speed. So:

• 返信が早い = the reply comes soon / is prompt

That is why 早い is correct here.

Yes, that is possible, but it has a slightly different feel.

• 返信が早い focuses on a characteristic: she is quick to reply
• 早く返信する focuses more directly on the action of replying quickly

So:

• 彼女はメッセージの返信が早いです = She is quick to reply to messages
• 彼女はメッセージに早く返信します = She replies quickly to messages

Both are natural, but 返信が早い is a very common and natural way to describe someone’s communication style.

That is a good question. い-adjectives can end a sentence by themselves:

• 早い。

But adding です makes the sentence polite:

• 早いです。

So:

• 早い = plain style
• 早いです = polite style

The adjective itself does not change here; です is added for politeness.

That is why the full sentence ends with 早いです.

Yes, it is basically natural.

A normal written version would be:

彼女は仕事が忙しくても、メッセージの返信が早いです。

A few notes:

• In normal Japanese writing, there are no spaces between words.
• 忙しくても is one chunk; learners sometimes first see it split up for study, but in real writing it is written together.
• The sentence sounds like: Even when she is busy with work, she still replies to messages quickly.

If you wanted a slightly different nuance, you could also say:

• 彼女は仕事が忙しくても、メッセージへの返信が早いです。

Using への makes the direction to messages a little more explicit, but メッセージの返信 is also understandable and natural enough in many contexts.

By itself, て often connects things, like and:

• 安くておいしい = cheap and tasty

But in 〜ても, the て is part of a bigger grammar pattern:

• て + も = even if / although

So here you should understand 忙しくても as one grammar unit, not as:

• busy and also

Instead, it means:

• even if she is busy
• although she is busy

This is why it is important to learn 〜ても as its own pattern.