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Shopify says its move of five Checkout Blocks extensions to Polaris web components cut their transferred bundle sizes by 40.5% to 84.4% and improved checkout-volume-weighted Extension Load Time (ELT) by 8% at P50 and 7% at P90. These are Shopify’s reported results from its own production upgrade, not an independent benchmark or a promise that every extension will see the same gains.
What changed in Shopify’s Checkout Blocks upgrade?
In a case study published July 16, 2026, Shopify Engineering describes moving five high-traffic Checkout Blocks extensions from the 2025-07 API version to 2026-01. The extensions had used React and Shopify’s legacy Remote UI bridge. The upgraded stack uses remote-dom, Preact, and Polaris web components, and the team rewrote the codebase in TypeScript. Shopify also reorganized shared code from extensions/core to extensions/core-next and changed supporting libraries for Markdown, Liquid parsing, and date utilities.
Shopify explains that remote-dom mirrors DOM nodes from an extension’s sandbox to the host. The legacy approach instead used React’s reconciler to translate a React component tree into the serialized Remote UI protocol. Preact retained a React-like hooks model with a smaller framework footprint, while Polaris web components replaced the Polaris React component tree. Shopify presents that rendering explanation as part of its own engineering account.
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The table reports Shopify’s production results. Bundle-size reductions are Shopify’s reported transferred-bundle changes; ELT figures are per-extension percentage changes at the 50th and 90th percentiles. Negative ELT values indicate reductions in load time. Shopify defines ELT as the time one extension takes to load on its own.
#1 Best Overall
| Extension | Transferred bundle size reduction | ELT change, P50 | ELT change, P90 |
|---|---|---|---|
| Payment icons | 84.4% | -11.7% | -11.9% |
| Static content | 54.1% | -6.5% | -3.8% |
| Custom field | 44.6% | -8.1% | -5.9% |
| Dynamic content | 45.2% | -8.2% | -7.9% |
| Line-item actions | 40.5% | -8.4% | -6.5% |
Across the extension family, Shopify summarizes the transferred bundle reductions as 40% to 85%. Its checkout-volume-weighted ELT improved by 8% at P50 and 7% at P90. The engineering article does not report an absolute checkout-time reduction or a conversion lift, so these figures should not be read as evidence of either.
Why did the bundle sizes fall?
Shopify says the 2026-01 remote-dom CLI imposed a 64 KB gzip limit for each extension bundle in this migration. Before the upgrade, the extensions were 300–356 KB raw, or roughly 100–112 KB gzipped. Two dependency choices helped address the constraint:
Rank #2
- Removing React reconciler: Moving to Preact let the team drop
react-reconciler, which Shopify estimates at approximately 89 KB. - Replacing a general-purpose Liquid library: Shopify replaced
liquidjs—approximately 73 KB according to the case study—with its smaller in-house Liquid parser, “droplet.” Merchants can write Liquid in block configurations, so Shopify needed to preserve that capability within the bundle budget.
Shopify’s current API-version documentation separately lists a maximum compiled bundle size of 64 KB for checkout UI extensions and 128 KB for full-page customer account extensions. The engineering article describes a gzip limit for this migration, while the API documentation uses the term “compiled bundle”; those measures should not be assumed to be interchangeable. Check the live Shopify API versioning documentation for current wording and limits.
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Shopify’s 2025-07 Checkout UI extension documentation identifies that version as the last API version supporting React-based UI components. The official 2025-07 documentation and migration guide say later versions use Polaris web components, with those components used by default in 2025-10 and later.
The 2025-07 documentation said app deployments would be blocked starting October 1, 2026 if they included extension versions earlier than 2026-01. That date has passed. Because deployment enforcement and supported versions can change, check Shopify’s current developer guidance before planning a release rather than treating that historical deadline as a statement of today’s enforcement status.
For the platform’s broader component-system context, Shopify describes Polaris as a web-component toolkit spanning Admin, Checkout, Customer Accounts, and other Shopify surfaces, and says web components remove React runtime overhead in UI extensions. That is Shopify’s platform guidance, not a separate performance study.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How did Shopify roll out the migration?
Shopify says it released the changes incrementally, at most one extension per day, beginning with the lowest-risk extension. The team monitored per-extension metrics and reports rolling back once after a date-picker collision, then fixing the issue forward the next day. It also describes checking Liquid parity and live configuration. These details are Shopify’s account of its rollout, not independent verification of reliability.
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Where Checkout Blocks fit
Checkout UI extensions can add elements such as product upsells, trust badges, and loyalty-program displays, according to Shopify’s Checkout UI extensions documentation. Those are examples of the platform’s use cases; they should not be confused with a list of the five extensions in this particular case study.
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