How to Reduce Outlook Indexer CPU Use on Large OST Files

When Outlook is open, you may notice the process SearchIndexer.exe or Microsoft Outlook Indexer using 50% to 100% of your CPU. This happens most often when your OST file, the offline copy of your mailbox, is larger than 10 GB. The Windows Search service continuously reindexes new and changed items, and a large OST file forces the indexer to scan more data, causing high CPU usage. This article explains why the indexer consumes CPU resources on large OST files and provides specific settings and methods to lower that usage without disabling search entirely.

Why the Outlook Indexer Causes High CPU Usage on Large OST Files

The Windows Search indexer, SearchIndexer.exe, scans Outlook items such as emails, calendar entries, contacts, and tasks. It creates a local index so that you can search within Outlook instantly. Each time you receive a new email, the indexer must update the index. With a large OST file, the indexer has more items to scan and more data to write to the index. This scanning and writing process consumes CPU cycles, especially during the initial indexing after a new Outlook profile is created or after a Windows update resets the index.

The OST file grows as you keep more emails, attachments, and calendar items synchronized from the Exchange server or Microsoft 365. By default, Outlook downloads all mail from all folders. If your mailbox contains years of email with large attachments, the OST file can exceed 20 GB or more. The indexer then must process every new email, every moved email, and every changed item across this large data set. The CPU usage spikes during peak email traffic or when the indexer rebuilds the index from scratch.

Another factor is a corrupted or fragmented index. When the index becomes damaged, the indexer tries repeatedly to reindex the same items, causing sustained high CPU usage. This is often visible in Task Manager as SearchIndexer.exe running at 50% or more for hours.

Steps to Reduce Outlook Indexer CPU Usage

The following methods reduce the load on the indexer by shrinking the data it must process or by excluding folders from indexing entirely. Apply them in the order listed for the best results.

Method 1: Shrink the OST File by Changing the Sync Slider

1. Open Outlook Account Settings
   In Outlook, go to File > Account Settings > Account Settings. Select your email account and click Change.
2. Adjust the Mail to Keep Offline Slider
   In the Offline Settings section, drag the slider to a lower value, such as 6 months or 3 months. Click Next and then Finish. Outlook will remove older items from the OST file and keep only the selected time range.
3. Restart Outlook
   Close and reopen Outlook. The OST file size will decrease over the next few minutes as Outlook removes old data. Check Task Manager to see if CPU usage drops.

Method 2: Exclude Outlook Data Folders from Windows Search

1. Open Indexing Options
   Press the Windows key and type Indexing Options. Click the result to open the dialog.
2. Modify the Indexed Locations
   Click Modify. In the list of locations, expand the drive where your OST file is stored, typically C:. Clear the check box next to the folder that contains your Outlook data files. The default location is C:\Users\[YourUsername]\AppData\Local\Microsoft\Outlook. Click OK.
3. Confirm the Exclusion
   In Indexing Options, click Advanced. On the File Types tab, ensure that .ost and .pst are not listed or are unchecked. Click OK. The indexer will stop scanning those folders, reducing CPU usage.

Method 3: Rebuild the Search Index

1. Open Indexing Options
   Press the Windows key and type Indexing Options. Click the result to open the dialog.
2. Go to Advanced Settings
   Click Advanced. Under Troubleshooting, click Rebuild. Confirm the prompt. This deletes the current index and rebuilds it from scratch. The process may take several hours, but CPU usage will be high only during the rebuild, not persistently.
3. Monitor CPU Usage
   Open Task Manager and watch SearchIndexer.exe. After the rebuild completes, CPU usage should drop to near zero when Outlook is idle.

Method 4: Disable Windows Search for Outlook Only via Registry

This method prevents Outlook items from being indexed at all. Search inside Outlook will use the slower but less CPU-intensive search built into Outlook itself.

1. Open Registry Editor
   Press Windows + R, type regedit, and press Enter. Click Yes if prompted by User Account Control.
2. Navigate to the Outlook Policy Key
   Go to HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Search. If the Search key does not exist, right-click Outlook, select New > Key, and name it Search.
3. Create the DisableSearchPolicy DWORD
   Right-click in the right pane, select New > DWORD (32-bit) Value, and name it DisableSearchPolicy. Double-click it and set Value data to 1. Click OK.
4. Restart Outlook
   Close and reopen Outlook. The Windows Search indexer will no longer process Outlook items. CPU usage from SearchIndexer.exe should drop immediately.

If Outlook Still Has High CPU Usage After the Main Fix

Outlook Indexer Still Runs at 100% CPU After Rebuilding the Index

If the indexer continues to use 100% CPU after a rebuild, the OST file may be corrupted. Run the Inbox Repair Tool, Scanpst.exe, located in C:\Program Files (x86)\Microsoft Office\root\Office16 or the equivalent path for your version. Close Outlook, open Scanpst.exe, browse to your OST file, and click Start. After the repair, restart Outlook and check CPU usage.

SearchIndexer.exe Uses High CPU Even When Outlook Is Closed

This usually means the indexer is still processing other file types, such as documents or photos. Open Indexing Options and click Modify. Remove locations like your Documents or Desktop folders from the indexed list. This reduces the overall load on the indexer.

High CPU Usage Returns After a Few Hours

A third-party add-in may be causing Outlook to constantly update items, triggering reindexing. Go to File > Options > Add-ins. Click Go next to COM Add-ins and clear all check boxes. Restart Outlook. If CPU usage drops, re-enable add-ins one by one to identify the offender.

Windows Search Indexer vs Outlook Instant Search: Key Differences

Item	Windows Search Indexer	Outlook Instant Search
Process name	SearchIndexer.exe	Outlook.exe (built-in search)
Index location	System-level index database	Within the OST file itself
CPU impact	High on large OST files	Low to moderate
Search speed	Fast across all indexed items	Slower for very large mailboxes
Can be disabled	Yes, per folder or via registry	Not separately disabled
Best for	Users who search across multiple apps	Users who search only within Outlook

By default, Outlook uses Windows Search Indexer for faster search. If you disable it via the registry method above, Outlook falls back to its own Instant Search, which uses less CPU but returns results more slowly on large OST files.

You can now reduce the CPU load caused by the Outlook indexer by shrinking the OST file, excluding folders from indexing, rebuilding a corrupted index, or disabling Windows Search for Outlook. Try the sync slider adjustment first because it addresses the root cause of a large OST file. If you need to keep all email offline, use the registry method to stop the indexer entirely. After applying any of these changes, verify the result in Task Manager and confirm that Outlook search still works as expected.