Google Tag Manager Costs 5.3 Points of Good LCP

Tag-managed sites are good on LCP about 5 points less often on mobile, yet they pass Core Web Vitals at nearly the same overall rate, because they tend to be better-maintained sites.

Where a tag manager costs you: loading and stability (mobile)

Good LCP with GTM

64.6%

Good LCP without

69.9%

Good CLS with GTM

85.3%

Good CLS without

89.3%

Source: PageSpeed Matters analysis of 9.5 million origins with CrUX field data, mobile 75th percentile, mid-2026.

The dip shows up in loading (down 5.3 points) and stability (down 4.0 points), exactly where tags injected after render would hurt.

Source: Google Tag Manager Costs 5.3 Points of Good LCP - PageSpeed Matters