Myth: Lab data predicts your field score

Lab LCP 1.4s, field LCP p75 3.9s. Same page, 2.5s apart.
Lab is one device, one network, cold or warm by config. Field is the distribution of every real visitor.
— Your p75 user is on a 3-year-old Android over LTE with a contended CPU; the lab runs a fixed throttle that rarely matches the actual tail.
— Lab is for debugging cause; field is for grading outcome. Optimizing to lab green while field stays red is chasing the wrong number.
Takeaway: lab and field LCP differed by 2.5s on the identical page.