No, pengaru's right. They're not mutually exclusive. For ex: making the site faster and more stable means I spend less time fixing and keeping things running and more time building new features.

But cost optimization doesn't make a site more stable.(Remember the context here)

In fact, adding another component to optimize the cost is adding another point of failure.

Nah. As someone who has gone from a 1 person project to ~10 people and funding, taking the time to tackle problems like this is critical - it frees up an order of magnitude of time later, helps make the product experience smoother, saves you real money that, when you're one person, really can make a difference.