diff --git a/packages/react-native/src/private/webapis/performance/Performance.js b/packages/react-native/src/private/webapis/performance/Performance.js
index 5bea95e2212..d6b2250b0c1 100644
--- a/packages/react-native/src/private/webapis/performance/Performance.js
+++ b/packages/react-native/src/private/webapis/performance/Performance.js
@@ -25,31 +25,26 @@ import {
   performanceEntryTypeToRaw,
   rawToPerformanceEntry,
 } from './internals/RawPerformanceEntry';
-import {warnNoNativePerformance} from './internals/Utilities';
+import {
+  getCurrentTimeStamp,
+  warnNoNativePerformance,
+} from './internals/Utilities';
 import MemoryInfo from './MemoryInfo';
 import ReactNativeStartupTiming from './ReactNativeStartupTiming';
 import NativePerformance from './specs/NativePerformance';
 import {PerformanceMark, PerformanceMeasure} from './UserTiming';
 
-declare var global: {
-  // This value is defined directly via JSI, if available.
-  +nativePerformanceNow?: ?() => number,
-};
-
-const getCurrentTimeStamp: () => DOMHighResTimeStamp =
-  NativePerformance?.now ?? global.nativePerformanceNow ?? (() => Date.now());
-
 export type PerformanceMeasureOptions =
-  | {
+  | $ReadOnly<{
       detail?: DetailType,
       start?: DOMHighResTimeStamp | string,
       duration?: DOMHighResTimeStamp,
-    }
-  | {
+    }>
+  | $ReadOnly<{
       detail?: DetailType,
       start?: DOMHighResTimeStamp | string,
       end?: DOMHighResTimeStamp | string,
-    };
+    }>;
 
 const ENTRY_TYPES_AVAILABLE_FROM_TIMELINE: $ReadOnlyArray<PerformanceEntryType> =
   ['mark', 'measure'];
@@ -155,7 +150,7 @@ export default class Performance {
       );
     } else {
       warnNoNativePerformance();
-      computedStartTime = performance.now();
+      computedStartTime = getCurrentTimeStamp();
     }
 
     return new PerformanceMark(resolvedMarkName, {
diff --git a/packages/react-native/src/private/webapis/performance/UserTiming.js b/packages/react-native/src/private/webapis/performance/UserTiming.js
index 09aaa140d38..c49542f4992 100644
--- a/packages/react-native/src/private/webapis/performance/UserTiming.js
+++ b/packages/react-native/src/private/webapis/performance/UserTiming.js
@@ -12,44 +12,45 @@
 
 import type {DOMHighResTimeStamp} from './PerformanceEntry';
 
+import {getCurrentTimeStamp} from './internals/Utilities';
 import {PerformanceEntry} from './PerformanceEntry';
 
 export type DetailType = mixed;
 
-export type PerformanceMarkOptions = {
+export type PerformanceMarkOptions = $ReadOnly<{
   detail?: DetailType,
   startTime?: DOMHighResTimeStamp,
-};
+}>;
 
 export type TimeStampOrName = DOMHighResTimeStamp | string;
 
-export type PerformanceMeasureInit = {
+export type PerformanceMeasureInit = $ReadOnly<{
   detail?: DetailType,
   startTime: DOMHighResTimeStamp,
   duration: DOMHighResTimeStamp,
-};
+}>;
 
 class PerformanceMarkTemplate extends PerformanceEntry {
   // We don't use private fields because they're significantly slower to
   // initialize on construction and to access.
-  _detail: DetailType;
+  __detail: DetailType;
 
   // This constructor isn't really used. See `PerformanceMark` below.
   constructor(markName: string, markOptions?: PerformanceMarkOptions) {
     super({
       name: markName,
       entryType: 'mark',
-      startTime: markOptions?.startTime ?? performance.now(),
+      startTime: markOptions?.startTime ?? getCurrentTimeStamp(),
       duration: 0,
     });
 
     if (markOptions) {
-      this._detail = markOptions.detail;
+      this.__detail = markOptions.detail;
     }
   }
 
   get detail(): DetailType {
-    return this._detail;
+    return this.__detail;
   }
 }
 
@@ -67,11 +68,11 @@ export const PerformanceMark: typeof PerformanceMarkTemplate =
   ) {
     this.__name = markName;
     this.__entryType = 'mark';
-    this.__startTime = markOptions?.startTime ?? performance.now();
+    this.__startTime = markOptions?.startTime ?? getCurrentTimeStamp();
     this.__duration = 0;
 
     if (markOptions) {
-      this._detail = markOptions.detail;
+      this.__detail = markOptions.detail;
     }
   };
 
@@ -81,7 +82,7 @@ PerformanceMark.prototype = PerformanceMarkTemplate.prototype;
 class PerformanceMeasureTemplate extends PerformanceEntry {
   // We don't use private fields because they're significantly slower to
   // initialize on construction and to access.
-  _detail: DetailType;
+  __detail: DetailType;
 
   // This constructor isn't really used. See `PerformanceMeasure` below.
   constructor(measureName: string, measureOptions: PerformanceMeasureInit) {
@@ -93,12 +94,12 @@ class PerformanceMeasureTemplate extends PerformanceEntry {
     });
 
     if (measureOptions) {
-      this._detail = measureOptions.detail;
+      this.__detail = measureOptions.detail;
     }
   }
 
   get detail(): DetailType {
-    return this._detail;
+    return this.__detail;
   }
 }
 
@@ -116,7 +117,7 @@ export const PerformanceMeasure: typeof PerformanceMeasureTemplate =
     this.__duration = measureOptions.duration;
 
     if (measureOptions) {
-      this._detail = measureOptions.detail;
+      this.__detail = measureOptions.detail;
     }
   };
 
diff --git a/packages/react-native/src/private/webapis/performance/internals/Utilities.js b/packages/react-native/src/private/webapis/performance/internals/Utilities.js
index 672fec1bb63..9ee6ba1573f 100644
--- a/packages/react-native/src/private/webapis/performance/internals/Utilities.js
+++ b/packages/react-native/src/private/webapis/performance/internals/Utilities.js
@@ -9,6 +9,7 @@
  */
 
 import warnOnce from '../../../../../Libraries/Utilities/warnOnce';
+import NativePerformance from '../specs/NativePerformance';
 
 export function warnNoNativePerformance() {
   warnOnce(
@@ -16,3 +17,11 @@ export function warnNoNativePerformance() {
     'Missing native implementation of Performance',
   );
 }
+
+declare var global: {
+  // This value is defined directly via JSI, if available.
+  +nativePerformanceNow?: ?() => number,
+};
+
+export const getCurrentTimeStamp: () => DOMHighResTimeStamp =
+  NativePerformance?.now ?? global.nativePerformanceNow ?? (() => Date.now());