I ran into the same performance issues when reviewing Next.js apps, so I made a tool that scans Next.js sites and tells you what's slow. It's basically a performance tool that works best with Next.js apps, and highlights things like slow LCP, heavy JavaScript, and third-party impact and gives you suggestions and prioritizes them. Here is the link if anyone wants to check it out:
https://www.nextperf.dev/
https://www.nextperf.dev/