Lightroom Classic was suddenly extremely slow on Windows 11 – uninstalling KB5121003 fixed it
I wanted to share this in case anyone else suddenly experiences extremely poor performance in Lightroom Classic on Windows 11.
Lightroom Classic had suddenly become extremely slow on my PC. Starting Lightroom normally took around 10–15 seconds, but it had increased to 50-60 seconds. AI masking was also much slower.
At first I assumed it was a Lightroom issue. I tried quite a few things:
- Reset Lightroom preferences
- Created a completely new catalog
- Disabled Lightroom plug-ins
- Tried Lightroom Classic 15.3.1 instead of 15.4.1
- Reinstalled Lightroom Classic 15.4.1
None of these made a meaningful difference. Even a completely new catalog with basically no edits was extremely slow.
My PC has 32 GB RAM, an AMD Ryzen 7 3800X, an NVIDIA GeForce RTX 5070 with 12 GB VRAM, and an NVMe SSD, so the hardware shouldn't have been struggling with this.
I then noticed that Windows had installed the KB5121003 Windows 11 security update on August 11/12. As a test, I uninstalled KB5121003, restarted the PC and repeated exactly the same Lightroom tests.
Lightroom immediately felt responsive again. Interestingly, Google Chrome also became noticeably faster when loading websites, so this doesn't appear to be limited to Lightroom on my system.
If your Lightroom Classic has suddenly become unusually slow after a recent Windows update, it may be worth checking whether KB5121003 was installed around the time the problem started.