Hidden bug in Outlook (classic) with local PST file and POP3/IMAP: Outlook makes Windows 11 search indexer cause high workload and SSD wear

dark-rider 60 Reputation points
2026-01-16T08:23:16.7266667+00:00

Hello,

after update from Windows 10 to 11, Windows search indexer has changed in so far that Outlook (classic) now already for months repeatedly makes search indexer cause high workload and SSD wear. This was not the case with Windows 10.

First, when the search index is completed for user files and for Outlook, in my case it has 3.5 GB for 250.000 user files and 100.000 Outlook elements (.pst file of about 10 GB). That's fine.

But then each time Outlook (classic) is started and new e-mails are received, the whole search index is reworked for 10-20 minutes and expands to 5.5, 7.5 or 9.5 GB or back - without reasonable bigger data changes in Outlook.

The same happens after each restart of Windows 11. Search index since then permanently changes between 5.5 to 9.5 GB - the regular 3.5 GB are seen no more, except you manually rebuild the whole index. But even after a manual rebuild, the problem occurs again.

The standard path to the search index file named Windows.db is:

C:\ProgramData\Microsoft\Search\Data\Applications\Windows
(hidden files have to be selected to be shown in Windows Explorer)

This very regularly causes high search indexer workload (CPU) and changes much data (SSD) which slows down the system (more or less, depending on the hardware capabilities), accelerates SSD wear and among other things makes differential/incremental backups very large.

Further checking showed that after each update of M365 incl. Outlook (classic), so for example to 2508, to 2509 and to 2510, the bug disappeared for 5-10 days, but then it came back in the same way as before. During these 5-10 days, Windows search was working fine as before, also within Outlook, including most current data, so the annoying reorganization of the search index after each start of Outlook (classic) and after each restart of Windows 11 is not needed for proper function of search.

Outlook (classic) obviously has a bug which causes this annoying behaviour. I reported it to "Microsoft Learn", where a so-called moderator (Microsoft external staff) confirmed it as a bug on Sep 17, 2025, but since then, it was not registered as a known issue, no bug fix appeared and no more answer from Microsoft.

Because the consequences of this bug are severe, but hidden (not easy to notice and not easy to assign to Outlook):

Could others here using Outlook (classic) with local PST file check for the dramatically changing size of Windows.db and for the CPU workload of Search Indexer in Windows task manager every time Outlook (classic) is started and has received new e-mails, and report their results here?

  • Here's a shortlist what to do to see this hidden Outlook bug:
  • Make sure you use Windows 11 (not 10) and Outlook classic (not new).
  • Make sure you download and store your e-mails locally in a PST file (not online on mailservers), and that the PST file has a reasonable size (for example some GB).
  • Open the following folder and observe the size of Windows.db within:
  • C:\ProgramData\Microsoft\Search\Data\Applications\Windows
  • (copy this path to Windows Explorer's location line or navigate manually after activating hidden files to be shown via view/show/hidden elements)
  • Open task manager and observe the CPU/SSD load of Search Indexer.
  • Open Outlook (classic) and download new e-mails. Observe Windows.db and Search Indexer as described above, for about 10-20 minutes.
  • Also after restarting Windows 11, observe Windows.db and Search Indexer as described above, for about 10-20 minutes.
Outlook | Windows | Classic Outlook for Windows | For home

2 answers

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  1. dark-rider 60 Reputation points
    2026-02-15T11:26:33.9+00:00

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  2. AI answer

    2026-01-16T08:23:36.3633333+00:00

    The issue you are experiencing with Outlook (classic) causing high workload on the Windows Search indexer after upgrading to Windows 11 is not uncommon. It appears that the interaction between Outlook and the search indexing service can lead to performance problems, particularly when dealing with large PST files and numerous indexed items.

    Potential Causes and Solutions:

    1. Index Size and Item Count: The performance of the Windows Search indexer can degrade when the number of indexed items exceeds certain thresholds. For typical users, indexing fewer than 30,000 items is manageable, but performance issues may arise when indexing more than 400,000 items. Since you mentioned having around 100,000 Outlook elements in a 10 GB PST file, this could be contributing to the issue.
    2. Rebuilding the Index: If the index size fluctuates significantly and you notice high CPU usage, it may help to rebuild the index manually. However, as you noted, the problem reoccurs shortly after rebuilding.
    3. Outlook Settings: Consider adjusting the synchronization settings in Outlook to limit the number of items being indexed. This can help reduce the workload on the indexer.
    4. Defragmenting the Index Database: If the index database is growing too large, you can try defragmenting it using administrative commands. This can help reclaim space and potentially improve performance.
    5. Monitoring Updates: Since you observed that updates to M365 temporarily resolved the issue, keeping Outlook updated may help mitigate the problem until a more permanent fix is provided by Microsoft.
    6. Feedback to Microsoft: Since this issue has been acknowledged by Microsoft, continuing to report your experiences may help escalate the need for a bug fix.

    Conclusion:

    While there is no definitive solution available at this time, monitoring your index size, adjusting settings, and keeping your software updated are recommended steps to help manage the performance issues you are facing with Outlook and the Windows Search indexer.

    References:

    AI-generated content may be incorrect. Read our transparency notes for more information.

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