GLM 5.3 Flash: Metal resident-Q2 prefill −6–8% from 0e9cc2d (attention masking fix), measured on M5 Max

**Setup:** MacBook Pro M5 Max 128 GB, `GLM-5.3-Flash-Q2.gguf` (resident, `--warm-weights`),
`ds4-server --ctx 393216 --vision GLM-5.3-Flash-Vision-Encoder.gguf --kv-disk-dir ... --kv-cache-cold-max-tokens 393216`,
server chat path, cold prefill on unique text (KV purged per block), server-reported `avg=`.

**Observation:** updating from `110afdd` to `6289c51` cost GLM cold prefill consistently at every depth,
while decode stayed flat. Interleaved A/B (3 rounds per condition, per-block checkout+binary+KV purge,
thermal settle + cooldowns; first round discarded as cold):

| depth | 110afdd | 6289c51 | delta |
| --- | ---: | ---: | ---: |
| 8K cold | 411 t/s | 381 t/s | −7.3% |
| 16K cold | 385 t/s | 362 t/s | −6.0% |
| 24K cold | 372 t/s | 341 t/s | −8.4% |
| 2K delta on warm 14K | 313 t/s | 278 t/s | −11% |
| decode 0–24K | 24.2–31.6 t/s | 24.2–30.4 t/s | flat/noise |

**Attribution:** direct two-commit differential, `0e9cc2d^` (=9d9e129) vs `0e9cc2d`
("Fix GLM attention masking and reductions across GPU backends"), 2 interleaved rounds each,
same harness:

| depth | 9d9e129 | 0e9cc2d | delta |
| --- | ---: | ---: | ---: |
| 8K cold | 415 t/s | 382 t/s | −8.0% |
| 16K cold | 386 t/s | 359 t/s | −7.0% |
| 24K cold | 376 t/s | 355 t/s | −5.5% |

That accounts for essentially the whole update delta. The commit message records the Strix
continued-prefill improvement (21→68 t/s) and unchanged official scores, but not a Metal cost,
so this may be an unnoticed side effect of the added reduction-storage synchronization /
padded-selection handling on Apple GPUs rather than an accepted trade-off.

Happy to run patches or diagnostics on this M5 Max config. (Same machine/config as the
measurements in #897/#898.)