[diffusion] doc: add --perf-dump-path section to profiling doc (#15533)

python/sglang/multimodal_gen/docs/profiling.md

@@ -58,6 +58,24 @@ Load and visualize trace files at:
 
 For large trace files, reduce `--num-profiled-timesteps` or avoid using `--profile-all-stages`.
 
+
+### `--perf-dump-path` (Stage/Step Timing Dump)
+
+Besides profiler traces, you can also dump a lightweight JSON report that contains:
+- stage-level timing breakdown for the full pipeline
+- step-level timing breakdown for the denoising stage (per diffusion step)
+
+This is useful to quickly identify which stage dominates end-to-end latency, and whether denoising steps have uniform runtimes (and if not, which step has an abnormal spike).
+
+Example:
+
+```bash
+sglang generate \
+  --model-path <MODEL_PATH_OR_ID> \
+  --prompt "<PROMPT>" \
+  --perf-dump-path perf.json
+```
+
 ## Nsight Systems
 
 Nsight Systems provides low-level CUDA profiling with kernel details, register usage, and memory access patterns.