Is lab data or field data more important for website performance?

You need both, but they answer different questions. Lab data comes from a controlled test and helps identify what may be slowing a page down. Field data summarizes how real Chrome users experienced the site. The Chrome User Experience Report uses a rolling 28-day window, and page-level data may be unavailable when a page is not public or does not have enough visits. Use lab data to investigate and test changes, then use field data to confirm whether the real experience improved over time. See the Chrome User Experience Report methodology for details.