Windows Update caused Extremely Slow Boot/Shutdown

Hi All...

I wonder if anyone can help me triage an issue with my PC. Let me break down my configuration, the symptoms, and what I see from basic triage...

I have a small desktop PC with the following hardware profile:-

Motherboard: Asus ROG Strix B760-I Gaming Wifi, Rev 1.xx, runnion AMI BIOS v1604

CPU: Intel Core i9-14900T

RAM: 64Gb DDR5 RAM

GPU: nVidia T600

Disk: 2 x Samsung 990 Pro PCIe 4.0 4Tb

From a software perspective, the machine is configured to dual-boot via the grub Linux bootloader, with the following software configuration:-

Windows 11 Pro 64-Bit - fully up-to-date according to Windows Update

Linux Mint 21.3 Virginia, 64-Bit

On Wednesday evening I made a software change to Windows 11, installing a copy of RealVNC Viewer [the X-Window client]. I had a weird issue with this, where the installer downloaded the software but then sat as if waiting for something.

I cancelled the install [without any form of adverse message], then rebooted back in to Windows and the installation completed smoothly.

I checked and there is a Restore Point for this change...

Rebooting my machine from Windows in to Linux, I immediately noticed a massive slow-down in the "shutdown" and "boot" process.

Previously I would expect this machine to boot in around 5 seconds - certainly less than 10.

As Windows closed down after my issuing "Restart", it took about 90 seconds for Windows to shut down.

As I checked, I've found that:-

From a power-on, it takes 1 min 25 seconds to get to the BIOS Logo, ahead of the grub bootloader menu

It takes a further 2 min 15 seconds to get to the Mint Linux login screen

If I go from power-on to grub loader and select Windows, it now takes about 4 minutes to get form grub to the Windows 11 login screen...

With one exception, I do not see any unusual Error Messages - the exception being that now, when I try to shut down or reboot from Mint, I get some kind of error message flash up on the screen, but it's far too fast to read. I'm working my way through the various Mint logs, but there are 20 different logs and I'm not entirely sure what I'm looking for. I've checked Windows Event Viewer and there's nothing obvious, at least to my eye.

There have been ZERO changes to my machine on the Mint Linux side of things.

As soon as I got my machine back to Windows, I checked Windows Update > Update History, where I see the following:-

2025-01 Cumulative Update for .NET Framework 3.5 and 4.8.1 for Windows 11, version 23H2 for X64 (KB5049624) [Success on 15-Jan]

2025-01 .NET 8.0.12 Security Update for x64 Client (KB505025) [Success on 15-Jan]

2025-01 Cumulative Update for Windows 11 Version 23H2 for x64-based Systems (KB5050021) [Success on 15-Jan]

2024-11 .NET 8.0.11 Update for x64 Client (KB5047489) [Success on 18-Dec-24]

Here's the weird thing...

If the change in system performance was caused by something that happened during the background Windows Update... then why would it cause my machine to take 90 seconds to get from POST to the grub bootloader menu? There's literally no OS on the hardware at that point in time?

I've used Samsung Magician to check the condition of my two M.2 drives - both are running current firmware, both are at good temps [35 centigrade]. The boot volume shows 6.1Tb written and "Good" Drive health [it doesn't get better than that] and my second physical drive shows the same health and temperature and 10.3Tb written.

The fact that behaviour is terrible from POST to grub, then from grub to [either] OS, suggests than this isn't OS related. But in that case maybe the Windows updates are mere coincidence?

I'm a home-based worker and this machine is responsible for my being able to draw a salary... I'm not keen to poke it too hard in case I break something... but, it is configured with Macrium reflect and I run daily backups at 09:00 local time, so I could perform a full restore of the machine back to the morning before the oddity, if I have to.

Can anyone think of actions I can take to triage this fault please?

Thanks in advance!