The servers are fine. The code looks fine. Everything was fine three months ago. That's the part that makes Node.js performance problems at scale so disorienting — the degradation is gradual enough that there's no obvious moment where something broke. Response times that were 80ms are now 340ms. The team adds a server instance, it helps for two weeks, then the times creep back up. Someone suggests rewriting the whole thing in Go. Someone else says the database is the problem. Nobody is completely wrong and nobody is completely right. I've been in this situation. More than once. And the thing I've learned is that Node.js APIs slow down at scale for specific, diagnosable reasons — not random ones, not mysterious ones — and most of them are fixable without switching languages or tripling your infrastructure budget. But you have to know where to look, because the symptoms rarely point directly at the cause.…