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𝐏𝐞𝐫𝐟𝐨𝐫𝐦𝐚𝐧𝐜𝐞 𝐭𝐞𝐬𝐭𝐢𝐧𝐠 in JavaScript is vital to ensure fast, efficient web applications. Key metrics include load time, time to interactive (TTI), JavaScript execution time, and memory usage. Tools like 𝐂𝐡𝐫𝐨𝐦𝐞 𝐃𝐞𝐯𝐓𝐨𝐨𝐥𝐬 help analyze performance by profiling and identifying bottlenecks, while 𝐋𝐢𝐠𝐡𝐭𝐡𝐨𝐮𝐬𝐞 automates audits for performance metrics like FCP and TTI. Libraries like 𝐁𝐞𝐧𝐜𝐡𝐦𝐚𝐫𝐤.𝐣𝐬 allow micro-benchmarking specific functions. Common issues include excessive DOM manipulation, large bundles, and memory leaks. Optimization techniques like 𝐜𝐨𝐝𝐞 𝐬𝐩𝐥𝐢𝐭𝐭𝐢𝐧𝐠, 𝐥𝐚𝐳𝐲 𝐥𝐨𝐚𝐝𝐢𝐧𝐠, 𝐦𝐢𝐧𝐢𝐟𝐢𝐜𝐚𝐭𝐢𝐨𝐧, and using 𝐂𝐃𝐍𝐬 enhance performance, ensuring smooth user experiences and improved SEO.