What is the difference between vertical and horizontal performance optimization strategies for a Node.js application, and how do you decide which to pursue first?

Vertical optimization improves the efficiency of the existing code and single-instance resource usage (fixing an N+1 query, adding caching, reducing unnecessary object allocation, optimizing a hot algorithm) -- generally the first place to look, since it often yields the biggest wins relative to effort and doesn't require added infrastructure. Horizontal optimization instead adds more capacity (more server instances, more database read replicas, more worker processes) to handle the same inefficient-per-unit workload at greater scale -- valuable once genuine vertical improvements have been exhausted or aren't cost-effective, but simply scaling horizontally without addressing an underlying inefficiency (like an N+1 query) just means paying more infrastructure cost to mask the same root problem.

// Vertical: fix the actual inefficiency first
// Before: N+1 queries per request
// After: a single batched/eager-loaded query per request -- same infrastructure, much faster

// Horizontal: only after vertical optimization is exhausted
// Add more server instances behind the load balancer to handle overall higher total traffic volume