diff --git a/lib/OnyxCache.ts b/lib/OnyxCache.ts
index e105a0ab6..7d68b4409 100644
--- a/lib/OnyxCache.ts
+++ b/lib/OnyxCache.ts
@@ -77,6 +77,7 @@ class OnyxCache {
             'set',
             'drop',
             'merge',
+            'hydrate',
             'hasPendingTask',
             'getTaskPromise',
             'captureTask',
@@ -199,6 +200,73 @@ class OnyxCache {
         OnyxKeys.deregisterMemberKey(key);
     }
 
+    /**
+     * Bulk-loads values into a cache that's expected to be empty, skipping merge()'s per-key clone when
+     * safe. Falls back to a real merge for any key that already has a value, in case the cache wasn't
+     * empty after all. Used only by `Onyx.init()`.
+     * @param data - a map of (cache) key - values
+     */
+    hydrate(data: Record<OnyxKey, OnyxValue<OnyxKey>>): void {
+        if (typeof data !== 'object' || Array.isArray(data)) {
+            throw new Error('data passed to cache.hydrate() must be an Object of onyx key/value pairs');
+        }
+
+        const affectedCollections = new Set<OnyxKey>();
+
+        // eslint-disable-next-line no-restricted-syntax, guard-for-in
+        for (const key in data) {
+            if (!Object.hasOwn(data, key)) {
+                continue;
+            }
+
+            const value = data[key];
+            this.addKey(key);
+
+            if (value === undefined) {
+                this.addNullishStorageKey(key);
+                continue;
+            }
+
+            const collectionKey = OnyxKeys.getCollectionKey(key);
+
+            if (value === null) {
+                this.addNullishStorageKey(key);
+                delete this.storageMap[key];
+
+                if (collectionKey) {
+                    affectedCollections.add(collectionKey);
+                }
+            } else {
+                this.nullishStorageKeys.delete(key);
+
+                const existing = this.storageMap[key];
+
+                if (existing !== undefined) {
+                    // Key already has a value, so the empty-cache assumption doesn't hold here - merge instead of clobbering.
+                    this.storageMap[key] = utils.fastMerge(existing, value, {
+                        shouldRemoveNestedNulls: true,
+                        objectRemovalMode: 'replace',
+                    }).result;
+                } else if (utils.needsNormalization(value)) {
+                    this.storageMap[key] = utils.fastMerge(undefined, value, {
+                        shouldRemoveNestedNulls: true,
+                        objectRemovalMode: 'replace',
+                    }).result;
+                } else {
+                    this.storageMap[key] = value;
+                }
+
+                if (collectionKey) {
+                    affectedCollections.add(collectionKey);
+                }
+            }
+        }
+
+        for (const collectionKey of affectedCollections) {
+            this.dirtyCollections.add(collectionKey);
+        }
+    }
+
     /**
      * Deep merge data to cache, any non existing keys will be created
      * @param data - a map of (cache) key - values
diff --git a/lib/OnyxUtils.ts b/lib/OnyxUtils.ts
index 31c50c38e..d13caa89d 100644
--- a/lib/OnyxUtils.ts
+++ b/lib/OnyxUtils.ts
@@ -1044,9 +1044,11 @@ function initializeWithDefaultKeyStates(): Promise<void> {
                 allDataFromStorage[key] = value;
             }
 
-            // Load all storage data into cache silently (no subscriber notifications)
+            // Load all storage data into cache silently (no subscriber notifications).
+            // hydrate() rather than merge(): the cache is empty at this point, so a per-key fastMerge
+            // would only deep-clone every row it was handed.
             cache.setAllKeys(Object.keys(allDataFromStorage));
-            cache.merge(allDataFromStorage);
+            cache.hydrate(allDataFromStorage);
 
             // For keys that have a developer-defined default (via `initialKeyStates`), merge the
             // persisted value with the default so new properties added in code updates are applied
diff --git a/lib/utils.ts b/lib/utils.ts
index 287f4cf26..45895be1a 100644
--- a/lib/utils.ts
+++ b/lib/utils.ts
@@ -196,6 +196,35 @@ function isMergeableObject<TObject extends Record<string, unknown>>(value: unkno
     return isNonNullObject && !(value instanceof RegExp) && !(value instanceof Date) && !Array.isArray(value);
 }
 
+/**
+ * Reports whether a value needs cleaning (nested null/undefined, or the replace-object mark) before it's
+ * safe to store by reference. Read-only, non-allocating.
+ */
+function needsNormalization(value: unknown): boolean {
+    if (value === null || value === undefined || typeof value !== 'object' || Array.isArray(value)) {
+        return false;
+    }
+
+    // eslint-disable-next-line no-restricted-syntax, guard-for-in
+    for (const key in value) {
+        if (key === ONYX_INTERNALS__REPLACE_OBJECT_MARK) {
+            return true;
+        }
+
+        const propertyValue = (value as Record<string, unknown>)[key];
+
+        if (propertyValue === null || propertyValue === undefined) {
+            return true;
+        }
+
+        if (typeof propertyValue === 'object' && !Array.isArray(propertyValue) && needsNormalization(propertyValue)) {
+            return true;
+        }
+    }
+
+    return false;
+}
+
 /** Deep removes the nested null values from the given value. Returns the original reference if no nulls were found. */
 function removeNestedNullValues<TValue extends OnyxInput<OnyxKey> | null>(value: TValue): TValue {
     if (value === null || value === undefined || typeof value !== 'object' || Array.isArray(value)) {
@@ -346,6 +375,7 @@ export default {
     isEmptyObject,
     formatActionName,
     removeNestedNullValues,
+    needsNormalization,
     checkCompatibilityWithExistingValue,
     pick,
     omit,