diff --git a/configurator/src/components/shell/PreviewPanel.svelte b/configurator/src/components/shell/PreviewPanel.svelte
index 06d61482..fee78000 100644
--- a/configurator/src/components/shell/PreviewPanel.svelte
+++ b/configurator/src/components/shell/PreviewPanel.svelte
@@ -481,70 +481,108 @@ ${BODIES[template]}
   let htmlLight = $derived(buildIframeHTML({}, "light", previewMotion, previewTemplate, frameworkCSSStatic));
   let htmlDark = $derived(buildIframeHTML({}, "dark", previewMotion, previewTemplate, frameworkCSSStatic));
 
+  // SL-020: the DOM writes below (style/attribute mutation on the iframe
+  // document, plus bumpPreviewVersion()'s resolveCache.clear()) are cheap
+  // individually but these effects re-run on every override tick — e.g. every
+  // input event while dragging a slider, many times per animation frame.
+  // rAF-coalescing collapses a burst of same-frame reruns into a single
+  // apply using the latest captured values ("coalesce to latest"), since only
+  // the frame actually painted to the user matters. The effect body above
+  // this comment still runs synchronously on every change (so Svelte's
+  // dependency tracking is unaffected) — only the DOM-writing work below is
+  // deferred. The `return () => cancelAnimationFrame(rafId)` cleanup fires
+  // before Svelte re-runs this effect (or on unmount), so a still-pending
+  // frame from a superseded run is cancelled before scheduling the next one.
   $effect(() => {
     const _ov = overrides;
     const _theme = previewTheme;
     const _count = loadCount;
     const _lock = lumlockerPreview.value;
 
-    const iframe = iframeEl;
-    if (_count === 0 || !iframe) return;
-    const doc = iframe.contentDocument;
-    if (!doc) return;
+    const rafId = requestAnimationFrame(() => {
+      const iframe = iframeEl;
+      if (_count === 0 || !iframe) return;
+      const doc = iframe.contentDocument;
+      if (!doc) return;
 
-    const styleEl = doc.getElementById("slashed-overrides");
-    if (styleEl) {
-      styleEl.textContent = generateCSS(withDerivedOverrides(_ov), { mode: "root", banner: false });
-    }
+      const styleEl = doc.getElementById("slashed-overrides");
+      if (styleEl) {
+        styleEl.textContent = generateCSS(withDerivedOverrides(_ov), { mode: "root", banner: false });
+      }
+
+      injectFontsIntoDoc(doc, _ov);
 
-    injectFontsIntoDoc(doc, _ov);
+      // Framework activates dark mode via [data-theme="dark"] (color-scheme +
+      // light-dark()), NOT a class — keep this in sync with buildIframeHTML.
+      doc.documentElement.setAttribute("data-theme", _theme);
 
-    // Framework activates dark mode via [data-theme="dark"] (color-scheme +
-    // light-dark()), NOT a class — keep this in sync with buildIframeHTML.
-    doc.documentElement.setAttribute("data-theme", _theme);
+      // Luminance lock preview — mirrors :root[data-lumlocker] in core/themes.css.
+      if (_lock) doc.documentElement.setAttribute("data-lumlocker", "");
+      else doc.documentElement.removeAttribute("data-lumlocker");
 
-    // Luminance lock preview — mirrors :root[data-lumlocker] in core/themes.css.
-    if (_lock) doc.documentElement.setAttribute("data-lumlocker", "");
-    else doc.documentElement.removeAttribute("data-lumlocker");
+      // This single-mode iframe is the canonical resolver source.
+      // registerPreviewDoc() always bumps internally (both its
+      // activeDoc-unchanged early-return and its replace-doc path do), so an
+      // explicit bumpPreviewVersion() here would double-bump/double-clear
+      // resolveCache for no reason.
+      registerPreviewDoc(doc);
+    });
 
-    // This single-mode iframe is the canonical resolver source.
-    registerPreviewDoc(doc);
-    bumpPreviewVersion();
+    return () => cancelAnimationFrame(rafId);
   });
 
   $effect(() => {
+    const _splitMode = splitMode;
     const _ov = overrides;
     const _lightCount = splitLightLoadCount;
     const _darkCount = splitDarkLoadCount;
     const _lock = lumlockerPreview.value;
-    const css = generateCSS(withDerivedOverrides(_ov), { mode: "root", banner: false });
 
-    const applyLock = (doc: Document) => {
-      if (_lock) doc.documentElement.setAttribute("data-lumlocker", "");
-      else doc.documentElement.removeAttribute("data-lumlocker");
-    };
-
-    if (splitLightEl && _lightCount > 0) {
-      const doc = splitLightEl.contentDocument;
-      if (doc) {
-        const styleEl = doc.getElementById("slashed-overrides");
-        if (styleEl) styleEl.textContent = css;
-        injectFontsIntoDoc(doc, _ov);
-        applyLock(doc);
-        // In split mode the light pane is the canonical resolver source.
-        registerPreviewDoc(doc);
+    // Split iframes only exist in the DOM under {#if splitMode} below — skip
+    // scheduling entirely rather than doing per-frame no-op work while the
+    // single-iframe effect above is the one actually driving the preview.
+    if (!_splitMode) return;
+
+    const rafId = requestAnimationFrame(() => {
+      const css = generateCSS(withDerivedOverrides(_ov), { mode: "root", banner: false });
+
+      const applyLock = (doc: Document) => {
+        if (_lock) doc.documentElement.setAttribute("data-lumlocker", "");
+        else doc.documentElement.removeAttribute("data-lumlocker");
+      };
+
+      // registerPreviewDoc() (light pane) already bumps internally; only
+      // bump explicitly when the dark pane was the sole one updated this
+      // frame, so consumers still get notified without double-bumping.
+      let lightApplied = false;
+      let darkApplied = false;
+
+      if (splitLightEl && _lightCount > 0) {
+        const doc = splitLightEl.contentDocument;
+        if (doc) {
+          const styleEl = doc.getElementById("slashed-overrides");
+          if (styleEl) styleEl.textContent = css;
+          injectFontsIntoDoc(doc, _ov);
+          applyLock(doc);
+          // In split mode the light pane is the canonical resolver source.
+          registerPreviewDoc(doc);
+          lightApplied = true;
+        }
       }
-    }
-    if (splitDarkEl && _darkCount > 0) {
-      const doc = splitDarkEl.contentDocument;
-      if (doc) {
-        const styleEl = doc.getElementById("slashed-overrides");
-        if (styleEl) styleEl.textContent = css;
-        injectFontsIntoDoc(doc, _ov);
-        applyLock(doc);
+      if (splitDarkEl && _darkCount > 0) {
+        const doc = splitDarkEl.contentDocument;
+        if (doc) {
+          const styleEl = doc.getElementById("slashed-overrides");
+          if (styleEl) styleEl.textContent = css;
+          injectFontsIntoDoc(doc, _ov);
+          applyLock(doc);
+          darkApplied = true;
+        }
       }
-    }
-    bumpPreviewVersion();
+      if (darkApplied && !lightApplied) bumpPreviewVersion();
+    });
+
+    return () => cancelAnimationFrame(rafId);
   });
 
   let isConstrained = $derived(previewWidth !== "fluid");