Severe lag, CPU spikes, and sustained heavy disk writes while editing (Lightroom 9.5.1, macOS, Apple Silicon)

Severe lag, CPU spikes, and sustained heavy disk writes while editing (Lightroom 9.5.1, macOS, Apple Silicon)

App version: Adobe Lightroom (cloud) 9.5.1
OS: macOS 26.6.2 (Build 25G83)
Hardware: MacBook Pro, Mac15,6, 18 GB RAM, Apple Silicon

Symptoms:
After just a few minutes of ordinary RAW editing (Leica DNG files), Lightroom becomes severely laggy. Sliders stop responding smoothly, preset previews don't render, and the system heats up with fans at full speed. This isn't tied to any heavy feature like Denoise or masking, it happens on basic actions.

Measured evidence:
- Lightroom's own CPU usage has been observed as high as 341% and 574% (Activity Monitor), with GPU usage spiking to 40%+ during simple actions like dragging one slider or switching to a new photo.
- macOS generated an automatic disk-writes resource-violation report during one editing session: Lightroom dirtied 2,147.67 MB of file-backed memory over 2,123 seconds, an average of 1,011.51 KB/sec, against the system's expected limit of 24.86 KB/sec. Incident ID: F6C37033-2424-4F81-955D-3D86DDB3357E.
- The Local Storage folder configured for originals/cache is empty despite this volume of writes, suggesting the writes are happening elsewhere internally (catalog database or app cache), not in the user-visible local storage location.

What I've already ruled out through testing:
- Memory pressure and swap (0 bytes swap at a clean baseline and during testing)
- A separate runaway Creative Cloud Core Service process (identified and fixed independently, unrelated to this issue)
- Cloud sync (paused syncing entirely, no change)
- Photo Cache size (raised from 8 GB to the 50 GB maximum, no change)
- GPU acceleration (disabling Use Graphics Processor made CPU usage worse, not better, so I re-enabled it)
- Preview generation (previews were already fully built, no "Building Previews" indicator was active during the lag)

This closely matches another report on this forum describing 100% CPU usage when opening presets or switching images on an Apple Silicon MacBook Pro running the cloud version of Lightroom.