There's nothing to decide. OP said they are not in a position of power to fire those employees.
I've been in this situation and no amount of taking to founders helped. Maybe OP is in a different position but unless they were brought in to fix something urgent, I doubt the founders actually think there's a problem at all.
In my situation, I realized I was fighting up and down with people that didn't see the problem or didn't care so I just stopped. There's no point in trying to "fix" something nobody sees as a problem. Either you compromise your quality and get with the program or you just quit.
@gtirloni can you share more about your experiences talking to founders? Did they just disagree with you that there was a problem at all?
The reason I sometimes join them is because I personally find it far more interesting than a corporate gig. The reason I'm here writing this comment and the reason I work in tech is because I've always been interested in tech startups.
The way I've always saw it is that typically within a year (or maybe two) the company will likely have failed and you can move on to something else, or if you're lucky the company will have found its market fit and you should then perhaps expect to be move fairly compensated for your work, and if you're not you'll have a lot of bargaining power within the company.
If you're very lucky, then you'll get to ride the coattails of the success of the company, both in terms of having a likely leadership position in the company and in terms of stock options.
It's high risk / high reward. Take it or leave it.
Lots of people like the idea of working for a startup, but very few are well suited for it. It's why I think probation periods for employees in startups are essential and if an employee underperforms in the first month they should be cut.
The place I'm working at right now is about as far as you can get from a startup, but simply managing crappy devs on our team takes up about half of my day. It's fine given what I'm doing, but I wouldn't personally be okay with wasting time like that at a startup, and I'd probably quit myself if it wasn't addressed.
But there are no hard rules here. The startup might be fine if the business plan is solid and rest of the team can compensate for the under performers. But either way you'd still be better off without them.
You'll have to decide if prioritising the success of the company is more important than being nice. I personally I wouldn't like the odds of a startup that's aware half of its employees are underperforming but can't do what's needed. And if you're building the company with other people's money I'd argue you're also being negligent with your investors' capital.