diff --git a/.changeset/lazy-surface-iframes.md b/.changeset/lazy-surface-iframes.md
new file mode 100644
index 0000000..452ed1f
--- /dev/null
+++ b/.changeset/lazy-surface-iframes.md
@@ -0,0 +1,5 @@
+---
+"sideshow": patch
+---
+
+Lazy-load sandboxed surface iframes to reduce initial viewer work on long sessions.
diff --git a/e2e/viewer.spec.ts b/e2e/viewer.spec.ts
index 5c8fa55..94d1020 100644
--- a/e2e/viewer.spec.ts
+++ b/e2e/viewer.spec.ts
@@ -101,6 +101,7 @@ test("resize bridge grows the iframe beyond its 120px default", async ({ page, s
   await page.goto(server.url);
   const iframe = page.locator(".card iframe");
   await expect(iframe).toBeVisible();
+  await expect(iframe).toHaveAttribute("loading", "lazy");
   // the sandboxed bridge must report content height via postMessage; this is
   // the WebKit-quirk regression test (see CLAUDE.md)
   await expect
diff --git a/viewer/src/Card.tsx b/viewer/src/Card.tsx
index 0f76934..bccbe2a 100644
--- a/viewer/src/Card.tsx
+++ b/viewer/src/Card.tsx
@@ -321,6 +321,7 @@ export function Card(props: { post: Post; standalone?: boolean }) {
                       });
                     }}
                     sandbox="allow-scripts"
+                    loading="lazy"
                     class={SURFACE_FRAME_CLASSES[surface.kind]}
                     title={
                       props.post.surfaces.length > 1