first-input-delay VS layout-instability

Since properly measuring the FID requires real user interaction, you can’t simply scan a website and get that value. However, you can add code to your application to get that information. Once you have, it’s up to you to do whatever you want with it. Google has an open-source script you can use directly inside your page to measure this metric: https://github.com/GoogleChromeLabs/first-input-delay

You can find it right under the TBT value. If on the other hand, you want to incorporate this value into a custom report or you want more control over how it gets calculated, you can take advantage of the Layout Instability API which gives you everything you need to measure how elements shift around on your webpage. This alternative gives you full control over the calculation and what to do with it, however, notice that it’s not a straightforward formula and that it requires lots of consideration (such as what happens with iFrames and pages on the background). So instead, if you're looking for a custom alternative you can check out Google’s WebVitals API which provides a very handy getCLS method.