diff --git a/.changeset/perf-avatarstack-has-selector.md b/.changeset/perf-avatarstack-has-selector.md
new file mode 100644
index 00000000000..cd9f6ea5715
--- /dev/null
+++ b/.changeset/perf-avatarstack-has-selector.md
@@ -0,0 +1,8 @@
+---
+'@primer/react': patch
+---
+
+perf(AvatarStack): Optimize CSS :has() selector and throttle MutationObserver
+
+- Scope `:has([data-square])` to direct child with `>` combinator for O(1) lookup
+- Throttle MutationObserver callback using requestAnimationFrame to batch DOM reads
diff --git a/packages/react/src/AvatarStack/AvatarStack.module.css b/packages/react/src/AvatarStack/AvatarStack.module.css
index b455ec447c8..5adc0fa958e 100644
--- a/packages/react/src/AvatarStack/AvatarStack.module.css
+++ b/packages/react/src/AvatarStack/AvatarStack.module.css
@@ -143,7 +143,8 @@
     box-shadow: 0 0 0 var(--avatar-border-width) transparent;
   }
 
-  &:not([data-component='Avatar']):not(:has([data-square])) {
+  /* PERFORMANCE: Avatar with data-square is direct child, scope with > for O(1) lookup */
+  &:not([data-component='Avatar']):not(:has(> [data-square])) {
     border-radius: 50%;
   }
 
diff --git a/packages/react/src/AvatarStack/AvatarStack.tsx b/packages/react/src/AvatarStack/AvatarStack.tsx
index ae400a90876..8d67d7249b0 100644
--- a/packages/react/src/AvatarStack/AvatarStack.tsx
+++ b/packages/react/src/AvatarStack/AvatarStack.tsx
@@ -120,7 +120,19 @@ const AvatarStack = ({
         setHasInteractiveChildren(hasInteractiveNodes(stackContainer.current))
       }
 
-      const observer = new MutationObserver(interactiveChildren)
+      // Track pending frame to throttle MutationObserver callbacks
+      let pendingFrame: number | null = null
+      const throttledInteractiveChildren = () => {
+        if (pendingFrame !== null) {
+          cancelAnimationFrame(pendingFrame)
+        }
+        pendingFrame = requestAnimationFrame(() => {
+          pendingFrame = null
+          interactiveChildren()
+        })
+      }
+
+      const observer = new MutationObserver(throttledInteractiveChildren)
 
       observer.observe(stackContainer.current, {childList: true})
 
@@ -129,6 +141,9 @@ const AvatarStack = ({
 
       return () => {
         observer.disconnect()
+        if (pendingFrame !== null) {
+          cancelAnimationFrame(pendingFrame)
+        }
       }
     }
   }, [])