Event loop lag is an essential, but often overlooked performance metric for Node.js applications. What is it and why does it matter?So I guess we can agree that synchronous == bad. The problem is that typically, you don’t know if there is synchronous code in your application. You only realize that something is fishy when your application’s performance starts degrading. One advice that many Node.js devs would give you in this situation is to start profiling and flame graphing your app. While profiling is indeed a great way to find the needle in the haystack I would argue that it should be the last resort as the process is difficult and time consuming. Furthermore, profiling is usually what you do once the damage is done and it’s too late.