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SQL 2025 slow with many active databases

Microsoft SQL Server 2025 currently has a bug that occurs around 500 active databases. This bug makes SQL server completely unusable. Microsoft has added multiple features in the 2025 release, but the 'persisted statistics for readable secondary replicas' feature causes this bug. The feature might refer to secondary replicas but it also occurs on single standalone server! Express, standard and enterprise are all affected by this bug.

Workaound

Microsoft has created a startup flag which can be enabed to disable this feature. This will also fix the bug and sql server will be usable and fast again. To set this startup flag, follow these steps:

  • Open the sql server configuration manager
  • Open the properties of your instance in the 'SQL Server Services' folder
  • Go to the 'Startup Parameters' tab
  • Add the following startup parameter: 
    -T15608 # Without spaces
  • Click on 'Ádd' and 'Apply' the changes
  • Restart the services on the 'Log On' tab.

Source

The bug is also documented on Microsoft Learn: - https://learn.microsoft.com/en-us/sql/sql-server/sql-server-2025-known-issues?view=sql-server-ver17#sql-server-might-become-slow-or-unresponsive-after-creating-or-bringing-online-a-large-number-of-databases