Lightroom Classic 15.4.1: export caps P-cores at 2.4GHz on M5 Pro (macOS 26)
# Environment
- Mac: MacBook Pro 14" 2026, M5 Pro (18-core) (identifier: Mac17,9)
- RAM: 64 GB
- macOS: 26.5.1 (build 25F80)
- Lightroom Classic: 15.4.1 (latest)
- Power: AC connected, High Power Mode enabled (powermode 2)
- Catalog: freshly created; source files and export destination both on internal SSD
- Previous machine for comparison: MacBook Pro (M1 Pro)
# Symptom
During export, total CPU utilization plateaus around 50% and exports are much slower
than expected. Using powermetrics, I confirmed that while exporting, the P-cores
(P0 cluster) are ~100% active but locked at roughly 2.4 GHz (2304-2652 MHz),
never boosting above 3 GHz. The P1 cluster stays mostly idle.
Measured comparison against M1 Pro (same photos, same export settings):
- Export: 92 s on M1 Pro -> 70 s on M5 Pro (only ~24% faster)
- AI Denoise: 9.5 s on M1 Pro -> 4.5 s on M5 Pro (~2.1x faster)
GPU/Neural Engine-bound tasks like Denoise scale as expected with the new chip,
while the CPU-bound export barely improves. Also, on the M1 Pro all cores were
fully utilized during export, whereas on the M5 Pro the P-cores are frequency-capped
as described above. This looks like a regression in export behavior across
chip generations.
# What I have ruled out
1. Thermal throttling: thermal pressure stays "Nominal" during export
(powermetrics --samplers thermal)
2. Low Power Mode: off (High Power Mode is enabled)
3. Background apps: reproduced with all third-party background apps quit
4. App Nap: disabled via NSAppSleepDisabled
5. taskpolicy -B applied to the Lightroom process: no change
6. Splitting the export into multiple simultaneous batches: CPU usage did not increase
7. Hardware sanity check: on the same machine, a synthetic CPU load test
unrelated to Lightroom (running 12 instances of "yes > /dev/null" in Terminal
to load all P-cores at 100%) boosts the P-cores to 4.2-4.4 GHz with thermal
pressure still Nominal. This confirms the chip, cooling, and power settings
are all healthy - the frequency cap only occurs during Lightroom exports.
# Conclusion (hypothesis)
The export worker threads appear to be created with a background/low QoS class,
so macOS runs them at reduced clock speeds even though the cores are available.
This matches reports in this thread:
https://community.adobe.com/t5/lightroom-classic-discussions/why-does-lightroom-not-use-the-performance-cores-m4max-during-export/td-p/15349358
# Request
Please review the QoS/priority of the export worker threads so that Apple Silicon
P-cores can run at their normal frequencies during export. I can provide full
powermetrics logs on request.
# Reference data
- During Lightroom export: P0 cluster averages ~2005-2449 MHz at 90-100% utilization
- Synthetic CPU load test: P0 cluster averages 3568 MHz (~53% residency at
4308-4380 MHz), P1 cluster averages 4002 MHz
AdChoices