diff --git a/.changeset/spinner-css-animation-sync.md b/.changeset/spinner-css-animation-sync.md
new file mode 100644
index 00000000000..1a47ed4d810
--- /dev/null
+++ b/.changeset/spinner-css-animation-sync.md
@@ -0,0 +1,5 @@
+---
+'@primer/react': patch
+---
+
+perf(Spinner): replace Web Animations API with CSS animation-delay sync
diff --git a/packages/react/src/Spinner/Spinner.tsx b/packages/react/src/Spinner/Spinner.tsx
index a212b580aa1..7b194238c79 100644
--- a/packages/react/src/Spinner/Spinner.tsx
+++ b/packages/react/src/Spinner/Spinner.tsx
@@ -1,6 +1,6 @@
 import {clsx} from 'clsx'
 import type React from 'react'
-import {useCallback, useEffect, useRef, useState, useSyncExternalStore} from 'react'
+import {useEffect, useState} from 'react'
 import {VisuallyHidden} from '../VisuallyHidden'
 import type {HTMLDataAttributes} from '../internal/internal-types'
 import {useId} from '../hooks'
@@ -8,6 +8,8 @@ import classes from './Spinner.module.css'
 import {useMedia} from '../hooks/useMedia'
 import {useFeatureFlag} from '../FeatureFlags'
 
+const ANIMATION_DURATION_MS = 1000
+
 const sizeMap = {
   small: '16px',
   medium: '32px',
@@ -37,18 +39,21 @@ function Spinner({
   ...props
 }: SpinnerProps) {
   const syncAnimationsEnabled = useFeatureFlag('primer_react_spinner_synchronize_animations')
-  const animationRef = useSpinnerAnimation()
+  const noMotionPreference = useMedia('(prefers-reduced-motion: no-preference)', false)
   const size = sizeMap[sizeKey]
   const hasHiddenLabel = srText !== null && ariaLabel === undefined
   const labelId = useId()
 
-  const [isVisible, setIsVisible] = useState(!delay)
+  const [{isVisible, syncDelay}, setVisibleState] = useState(() => ({
+    isVisible: !delay,
+    syncDelay: !delay ? computeSyncDelay() : 0,
+  }))
 
   useEffect(() => {
     if (delay) {
       const delayDuration = typeof delay === 'number' ? delay : delay === 'short' ? 300 : 1000
       const timeoutId = setTimeout(() => {
-        setIsVisible(true)
+        setVisibleState({isVisible: true, syncDelay: computeSyncDelay()})
       }, delayDuration)
 
       return () => clearTimeout(timeoutId)
@@ -59,11 +64,13 @@ function Spinner({
     return null
   }
 
+  const shouldSync = syncAnimationsEnabled && noMotionPreference
+  const mergedStyle = shouldSync ? {...style, animationDelay: `${syncDelay}ms`} : style
+
   return (
     /* inline-flex removes the extra line height */
     <span className={classes.Box}>
       <svg
-        ref={syncAnimationsEnabled ? animationRef : undefined}
         height={size}
         width={size}
         viewBox="0 0 16 16"
@@ -72,7 +79,7 @@ function Spinner({
         aria-label={ariaLabel ?? undefined}
         aria-labelledby={hasHiddenLabel ? labelId : undefined}
         className={clsx(className, classes.SpinnerAnimation)}
-        style={style}
+        style={mergedStyle}
         {...props}
       >
         <circle
@@ -99,127 +106,17 @@ function Spinner({
 
 Spinner.displayName = 'Spinner'
 
-type Subscriber = () => void
-
-type AnimationTimingValue = {
-  startTime: CSSNumberish | null
-}
-
-type AnimationTimingStore = {
-  subscribers: Set<Subscriber>
-  value: AnimationTimingValue
-  update(startTime: CSSNumberish): void
-  subscribe(subscriber: Subscriber): () => void
-  getSnapshot(): AnimationTimingValue
-  getServerSnapshot(): AnimationTimingValue
-}
-
-const animationTimingStore: AnimationTimingStore = {
-  subscribers: new Set<() => void>(),
-  value: {
-    startTime: null,
-  },
-  update(startTime) {
-    const value = {
-      startTime,
-    }
-    animationTimingStore.value = value
-    for (const subscriber of animationTimingStore.subscribers) {
-      subscriber()
-    }
-  },
-  subscribe(subscriber) {
-    animationTimingStore.subscribers.add(subscriber)
-    return () => {
-      animationTimingStore.subscribers.delete(subscriber)
-    }
-  },
-  getSnapshot() {
-    return animationTimingStore.value
-  },
-  getServerSnapshot() {
-    return animationTimingStore.value
-  },
-}
-
 /**
- * A utility hook for reading a common `startTime` value so that all animations
- * are in sync. This is a global value and is coordinated through `useSyncExternalStore`.
+ * Computes a negative animation-delay so all spinners land at the same
+ * rotation angle regardless of when they mount. Because every instance
+ * references the same clock (performance.now()), the CSS animation engine
+ * keeps them visually in sync without any Web Animations API calls
+ * (getAnimations, element.animate, startTime), which are significantly
+ * slower in Safari/WebKit.
  */
-function useAnimationTiming() {
-  return useSyncExternalStore(
-    animationTimingStore.subscribe,
-    animationTimingStore.getSnapshot,
-    animationTimingStore.getServerSnapshot,
-  )
-}
-
-/**
- * Uses a technique from Spectrum to coordinate animations:
- * @see https://github.com/adobe/react-spectrum/blob/ab5e6f3dba4235dafab9f81f8b5c506ce5f11230/packages/%40react-spectrum/s2/src/Skeleton.tsx#L21
- */
-function useSpinnerAnimation() {
-  const ref = useRef<Animation | null>(null)
-  const noMotionPreference = useMedia('(prefers-reduced-motion: no-preference)', false)
-  const animationTiming = useAnimationTiming()
-  return useCallback(
-    (element: HTMLElement | SVGSVGElement | null) => {
-      if (!element) {
-        return
-      }
-
-      if (ref.current !== null) {
-        return
-      }
-
-      if (noMotionPreference) {
-        const cssAnimation = element.getAnimations().find((animation): animation is CSSAnimation => {
-          if (animation instanceof CSSAnimation) {
-            return animation.animationName.startsWith('Spinner') && animation.animationName.endsWith('rotate-keyframes')
-          }
-          return false
-        })
-        // If we can find a CSS Animation, pause it and we will use the Web
-        // Animations API to pick up from where it left off
-        cssAnimation?.pause()
-
-        ref.current = element.animate(
-          [
-            {
-              transform: 'rotate(0deg)',
-            },
-            {
-              transform: 'rotate(360deg)',
-            },
-          ],
-          {
-            // var(--base-duration-1000)
-            duration: 1000,
-            // var(--base-easing-linear)
-            easing: 'cubic-bezier(0,0,1,1)',
-            iterations: Infinity,
-          },
-        )
-
-        // When the `startTime` value from `animationTimingStore` is `null` we
-        // are currently hydrating on the client. In this case, the first
-        // spinner to mount will set the `startTime` for all other spinners.
-        if (animationTiming.startTime === null) {
-          const startTime = cssAnimation?.startTime ?? 0
-
-          animationTimingStore.update(startTime)
-
-          // We use `startTime` to sync different animations. When all animations
-          // have the same startTime they will be in sync.
-          // @see https://developer.mozilla.org/en-US/docs/Web/API/Animation/startTime#syncing_different_animations
-          ref.current.startTime = startTime
-        } else {
-          ref.current.startTime = animationTiming.startTime
-        }
-      }
-    },
-    [noMotionPreference, animationTiming],
-  )
+function computeSyncDelay(): number {
+  const now = typeof performance !== 'undefined' ? performance.now() : 0
+  return -(now % ANIMATION_DURATION_MS)
 }
 
 export default Spinner