Cumulative Layout Shift is the one
A widget that appears after paint pushes content down, and that shift is measured. Reserving the container height solves most of it.
LCP rarely, INP sometimes
LCP only if the widget is the largest element above the fold. INP if the script blocks the main thread — a loading question, not a size one.
Measure before blaming
A page with an unoptimised hero image and four tracking scripts has poor vitals regardless. Run it with and without the widget; it takes two minutes.
Lab versus field
Synthetic scores are for diagnosis. Field data in Search Console is what counts.
Read the full version
The full write-up is in review widgets and page speed.