Shopify moves Checkout Blocks to Polaris Web Components, cutting bundles up to 85%

Shopify rebuilt five high-traffic Checkout Blocks extensions, moving from React and the Remote UI bridge to remote-dom with Preact and Polaris web components. Bundle sizes dropped 40–85% and load times fell about 8%; from October 1, 2026, deployments with extension versions older than API 2026-01 are blocked.

• Extension bundle sizes fell 40–85%, with payment-icons down 84.4%
• Extension load time dropped ~8% at the median and 7% at p90
• The 2026-01 CLI enforces a 64KB gzip budget per extension, down from 100–112KB
• From October 1, 2026, deployments with extensions older than API 2026-01 are blocked