Chrome Aurora team: Google Tag Manager with 18 tags increases Total Blocking Time ~20×

Claim: Per the Chrome Aurora team, a Google Tag Manager container with 18 tags increases Total Blocking Time nearly 20× vs a baseline page.

Source: Chrome Aurora team via corewebvitals.io.

Confidence: Industry-consensus.

Implication: The single largest avoidable INP cost is third-party JavaScript through tag managers. Most SMB sites pick up 8-15 tags over a 2-3 year period (GA, ads pixels, chat, A/B, heatmaps, retargeting, conversion tracking) without ever auditing the cumulative cost.

Operational pattern: defer/delay 3rd-party scripts until interaction via @next/third-parties or Partytown (web worker); quarterly tag audit; one-tag-in, one-out rule. See RULE: Defer or delay every third-party script until user interaction, except those required for first paint.