Summary

Landscape AI masking in Lightroom Classic 15.5/15.5.1 progressively becomes extremely slow during normal use on my system.

Immediately after a Windows reboot, a Landscape mask typically completes in approximately 6–7 seconds. After continued Lightroom use, the same operation on comparable images can take 60–90+ seconds.

I captured a controlled example in Lightroom Classic 15.5.1:

Degraded state before Windows reboot: 86.68 seconds
Normal state immediately after Windows reboot: 6.51 seconds

This represents approximately a 13.3× slowdown.

Restarting Lightroom does not consistently restore normal performance. Rebooting Windows restores the fast Landscape masking performance.

The problem appears specific to, or particularly severe with, Landscape AI masking. Other AI masks such as Sky and Subject do not exhibit the same degree of degradation in my testing.

System Configuration

Operating System: Windows 11
CPU: Intel Core i9-14900K
RAM: 64 GB
GPU: AMD Radeon RX 9070, 16 GB dedicated VRAM
AMD driver: 32.0.31044.16
Lightroom Classic: 15.5.1

The problem was also observed with Lightroom Classic 15.5.

I also have Lightroom Classic 14.5.2 installed separately on the same computer for comparison. Landscape masking in 14.5.2 remains substantially faster and does not exhibit the same severe degradation in my testing.

Steps to Reproduce

1. Reboot Windows.
2. Launch Lightroom Classic 15.5.1.
3. Open a RAW landscape photograph that has not previously had a Landscape AI mask generated.
4. Select Masking → Landscape and allow Lightroom to identify the landscape features.
5. Record the processing time.
6. Continue normal editing and perform Landscape masking on additional previously unanalyzed images.
7. Eventually Landscape masking becomes dramatically slower.
8. Test additional new images once the slowdown occurs.
9. Close and restart Lightroom and retest.
10. Reboot Windows, reopen Lightroom 15.5.1, and repeat the Landscape mask test.

Expected Result

Landscape AI masking should continue to complete in approximately the same amount of time throughout a Lightroom session, allowing for normal differences in image complexity.

Actual Result

Landscape masking initially completes in roughly 6–10 seconds, but after Lightroom has been used for some time, processing can increase to 60–90+ seconds per image.

Once the degraded state occurs, additional new images also process very slowly.

A Windows reboot restores Landscape masking to approximately 6–7 seconds.

Controlled Before/After Example

I captured Windows Task Manager GPU information while Lightroom Classic 15.5.1 was exhibiting the problem.

Before reboot — degraded state

Landscape masking required:

86.68 seconds

During the operation, Windows Task Manager showed sustained very high activity on the Radeon RX 9070 3D GPU engine for most of the Landscape analysis.

At completion, Task Manager showed approximately:

Dedicated GPU memory: 11.4 / 16.0 GB
Shared GPU memory: 1.7 / 47.7 GB
Total GPU memory: approximately 13.1 GB

Screenshot attached showing the 86.68-second processing time and GPU activity.

Screenshot also attached showing GPU status following the slow Landscape operation.

After Windows reboot — normal state

Without changing Lightroom versions, hardware, or GPU drivers, I rebooted Windows and repeated the test.

Landscape masking completed in:

6.51 seconds

The GPU graph shows a short burst of GPU activity rather than the prolonged high GPU utilization seen during the degraded 86.68-second operation.

Screenshots are attached showing the 6.51-second Landscape operation and GPU status following the operation.

The only significant intervention between the 86.68-second test and the 6.51-second test was rebooting Windows.

Additional Testing and Observations

I have spent considerable time trying to isolate this problem.

Performance progressively degrades

Landscape masking can initially operate normally and then become progressively slower during continued Lightroom use.

In previous testing I have observed Landscape operations progressing from approximately:

10 seconds → 34 seconds → 54–72 seconds

on previously unanalyzed images.

In another controlled comparison, Landscape masking measured:

6.5 seconds immediately after reboot vs. approximately 48 seconds after the problem developed.

The current screenshots demonstrate an even more severe example:

6.51 seconds vs. 86.68 seconds.

Previously analyzed images can behave differently

Once Lightroom has analyzed an image for Landscape masking, repeating the Landscape operation on that image can sometimes complete in only a few seconds.

Therefore, I have tried to use previously unanalyzed images when testing the underlying Landscape AI processing performance.

This distinction may be important when attempting to reproduce the issue.

Other AI operations are affected to some extent

When the degraded state develops, I have also observed substantially slower AI-update operations associated with other Lightroom functions.

For example, in one test:

AI update following Remove: approximately 26 seconds normally vs. 70 seconds degraded.

AI update following Denoise: approximately 20.5 seconds normally vs. 78.5 seconds degraded.

However, Landscape masking is the most obvious and repeatable manifestation of the problem.

Sky and Subject masks remain fast

During testing, Select Sky and Select Subject continued to perform relatively normally even when Landscape masking had become extremely slow.

This suggests the problem is not simply that all Lightroom AI masking becomes slow.

GPU acceleration test

I also tested Lightroom with GPU acceleration disabled.

Disabling GPU acceleration did not resolve the performance problem. In one test the processing times were approximately 90 seconds with GPU acceleration vs. 88 seconds without it.

Lightroom Classic 14.5.2 comparison

I installed Lightroom Classic 14.5.2 alongside the current version specifically for comparison.

On the same Windows computer, same Radeon RX 9070, and same image library, Landscape masking in Lightroom Classic 14.5.2 remains fast and does not exhibit the severe progressive slowdown I am seeing in Lightroom Classic 15.5/15.5.1.

This makes the behavior appear to be a regression introduced somewhere after Lightroom Classic 14.5.2.

GPU / Memory Observation

There appears to be a relationship between the performance degradation and changes in Lightroom's GPU workload and GPU-memory behavior.

After a Windows reboot, Landscape masking is fast and GPU activity associated with the operation is brief.

Once the problem develops, Landscape masking can cause sustained high GPU activity for a minute or longer.

I have also observed differences in dedicated versus shared GPU-memory usage between normal and degraded states.

I am not claiming that this proves a VRAM leak or that GPU memory management is necessarily the root cause. I am including the information because the GPU behavior changes significantly when the Landscape masking slowdown occurs and may help Adobe reproduce or diagnose the issue.

The fact that rebooting Windows restores normal performance, while restarting Lightroom does not consistently do so, may also be diagnostically significant.

Attached Screenshots

1. Slow Landscape Masking — before reboot
Lightroom Classic 15.5.1 Landscape AI operation taking 86.68 seconds, with Windows Task Manager showing sustained Radeon RX 9070 GPU activity.

2. GPU status following slow Landscape masking
GPU and GPU-memory state after the degraded Landscape operation.

3. Fast Landscape Masking — after Windows reboot
Same Lightroom Classic 15.5.1 installation after reboot. Landscape masking completes in 6.51 seconds, with Task Manager GPU activity visible.

4. GPU status following fast Landscape masking
GPU and GPU-memory state following the normal-speed Landscape operation.