What the tags cost, measured
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Time spent loading pages, per funnel walk
Each dot is one walk through the funnel (home → product → cart → checkout → purchase): the sum of the five page-load times. The line marks the median.
Page by page (medians)
This is the best case for server-side tagging: version A runs every tag in the browser and version B runs none there. Real setups usually keep a few tags in the browser, so real-world gains are smaller. All tag IDs are fake, so vendors reject the hits, but the browser still downloads and runs every vendor's code, which is the cost being measured.