Windows defender in Windows 25h2 with internal administrator account make defender very slow to open

[gilmar santos]

I recently installed Windows 11 25H2 and noticed a strange issue when enabling the built-in Administrator account.

When I activate the built-in Administrator account and log in with it, Windows Defender (Windows Security) becomes extremely slow to open. After clicking it, the window stays completely white for up to 10 seconds before finally loading and displaying the information.

However:

If I create a new user and add it to the Administrators group, Windows Defender opens normally.

The issue only happens when using the built-in Administrator account.

If I disable the built-in Administrator account and use a regular user with administrative privileges, everything works correctly.

I also tested the following:

I created a new user and added it to the Administrators group → Windows Defender opens normally.

Then I disabled UAC (User Account Control) for this new administrator account to avoid elevation prompts.

After disabling UAC, Windows Defender becomes slow again and shows the same white screen delay (just like when using the built-in Administrator account).

So it seems the issue is related to running without UAC elevation prompts.

Additionally:

On Windows 11 23H2, this problem does not occur.

The issue only started happening from version 24H2 onward (including 25H2).

Summary of behavior:

New admin user with UAC enabled → Windows Defender opens normally

Built-in Administrator account (UAC effectively disabled) → Defender very slow (white screen ~10 seconds)

New admin user with UAC disabled → Same slowdown occurs

Windows 11 23H2 No issue

Windows 11 24H2 and 25H2 → Issue occurs

command used: reg add "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System" /v EnableLUA /t REG_DWORD /d 0 /f


What I tested:
1 - I installed it on a virtual machine, everything worked perfectly.
2 - I disabled the hard drives in the BIOS to check if they were the problem: it didn't solve it.

I tried using DISM to check and nothing solved it.

I have another PC at work with an i5 11400 SSD and the same problem occurs; simply enabling the administrator account or disabling UAC makes Defender slow.


Has anyone experienced this behavior starting from 24H2?

Is Windows Defender now dependent on UAC being enabled for proper performance?

[binaryzero]

Assume yes.

Disable the builtin Administrator (well known SID, 500..), Create a new account, add to local Administrators group.