React Performance: JS Perf — Index Map Lookup. Keywords: map index lookup find. Platform: React/Next.js. Description: Build Map for repeated lookups instead of multiple .find() calls. Do: Build index Map for O(1) lookups. Don't: Use .find() in loops. Good Example: const byId = new Map(users.map(u => [u.id, u])); byId.get(id). Bad Example: users.find(u => u.id === order.userId) // O(n) each time. Severity: Low-Medium.