uri cheryeogi joheunikka jigeum sokdoreul jogeum ollija.

Because Korean often drops 의 in possessive phrases, especially in everyday speech.

So:

• 우리의 체력 = our stamina
• 우리 체력 = our stamina

Both are correct, but 우리 체력 sounds more natural and conversational here.

Also, 우리 in this sentence is functioning like our, not we.
If the speaker wanted to say as for us, they could say 우리는 instead.

체력 means physical strength, stamina, or endurance.

It is not exactly the same as:

• 힘 = strength, power, force
• 에너지 = energy

So 체력이 좋다 means something like:

• to have good stamina
• to be in good physical condition
• to have strong endurance

In this sentence, the idea is that the group is physically capable of going faster.

Because 체력 is the subject of 좋다.

The structure is:

• 우리(의) 체력 = our stamina
• 체력이 좋다 = stamina is good

So 체력이 marks 체력 as the thing being described by 좋다.

A very literal breakdown is:

• 우리 체력이 좋으니까 = because our stamina is good

-으니까 / -니까 means because, since, or so.

Here:

• 좋다 → 좋으니까
• 좋으니까 = because it is good / since it is good

So the first part means:

• 우리 체력이 좋으니까 = because we have good stamina

This ending is often used when the speaker gives a reason for a suggestion, command, or decision.

This is a very common learner question.

In Korean, -아/어서 is generally not used before a suggestion like -자.
But -으니까 works very naturally with suggestions, commands, and proposals.

So:

• 우리 체력이 좋으니까 지금 속도를 조금 올리자 = natural
• 우리 체력이 좋아서 지금 속도를 조금 올리자 = unnatural in standard grammar

That is one big reason -으니까 is used here.

It also gives a slight feeling of since that's the case, let's...

Literally, it means to raise the speed.

• 속도 = speed
• 속도를 = speed + object marker
• 올리다 = to raise, increase

So 속도를 올리다 means:

• to increase the speed
• to speed up
• to pick up the pace

In English, we would usually translate it more naturally as let's speed up a little.

Because 올리다 is a verb that takes an object.

You are raising/increasing something, and that something is speed.

So:

• 속도를 올리다 = increase the speed

The object marker -를 shows what is being increased.

조금 means a little or slightly.

It makes the suggestion softer and more moderate:

• 속도를 올리자 = let’s speed up
• 속도를 조금 올리자 = let’s speed up a little

So the speaker is not suggesting a huge jump in speed, just a small increase.

In casual speech, you will also often hear 좀 instead of 조금.

-자 means let’s.

So:

• 올리자 = let’s raise it / let’s increase it / let’s speed up

This is a casual or plain-style suggestion. It is natural among:

• friends
• teammates
• family
• people speaking informally

A more polite version could be:

• 올리죠
• 올립시다

So this sentence sounds like something one person says to their group in a casual setting.

Yes, it would be okay.

• 우리 체력이 좋으니까...
• 우리는 체력이 좋으니까...

Both can work, but the nuance is a little different.

우리 체력이 좋으니까 focuses directly on our stamina.

우리는 체력이 좋으니까 adds an explicit topic, something like:

• as for us, since we have good stamina...

So the version without 는 is a bit leaner and more conversational in this context.

Yes, most likely.

In this sentence, 우리 is naturally understood as we/us including you.
The speaker is talking to someone who is part of the group:

• our stamina is good
• so let’s speed up a little

That matches how -자 works, since let’s normally includes both the speaker and the listener.

Yes, basically now or right now.

Here it means something like:

• at this moment
• at this point
• now that we’re doing this

So the sentence is not just about general ability. It means:

• since our stamina is good, let’s increase the speed now

It gives the suggestion an immediate, in-the-moment feeling.

Yes, it is very natural.

It sounds like something someone might say while:

• running
• hiking
• biking
• training
• doing any group physical activity

The flow is very normal:

1. state the reason: 우리 체력이 좋으니까
2. make the suggestion: 지금 속도를 조금 올리자

So it feels like a realistic spoken sentence.