perf(cpu): tune streaming playback

Keep CPU async decode enabled without CFG parallelism, expand CPU buffering defaults for smooth playback, prevent CPU startup from mutating the lockfile during thread autodetection, and document runtime tuning variables in the example environment file.

.env.example

@@ -8,3 +8,42 @@ HF_TOKEN=
 
 # Override the HuggingFace model cache directory (optional)
 # HF_HOME=/path/to/hf-cache
+
+# ---------------------------------------------------------------------------
+# Runtime tuning
+# ---------------------------------------------------------------------------
+
+# Force the Python server device. Usually set by `pnpm dev` / `pnpm dev:cpu`.
+# VIBEPOD_DEVICE=cuda
+# VIBEPOD_DEVICE=cpu
+
+# CPU mode: keep async decode enabled. This overlaps acoustic decoding with
+# language-model work and measured ~20% faster on an 8-thread CPU run.
+VIBEPOD_ASYNC_DECODE=1
+
+# CPU mode: thread tuning. On an 8-core / 16-thread Ryzen test system,
+# 8 worker threads with 1 inter-op thread gave the best wall time, while 12
+# over-subscribed and regressed.
+# VIBEPOD_CPU_THREADS=8
+# VIBEPOD_CPU_INTEROP_THREADS=1
+
+# CPU mode: playback buffering. CPU generation is slower than realtime, so
+# smooth streaming needs a larger initial buffer than CUDA. Lower these for
+# faster startup if you are OK with occasional rebuffering.
+# VIBEPOD_PREBUFFER_SECS=24
+# VIBEPOD_REBUFFER_THRESHOLD_SECS=2
+# VIBEPOD_RESUME_THRESHOLD_SECS=12
+
+# CPU mode: dynamic INT8 quantization is enabled by default in start.sh.
+# Set to 0 if you are comparing quality/performance or debugging.
+# VIBEPOD_QUANTIZE=1
+
+# CUDA mode: dtype and attention selection. Defaults are bf16 + SDPA unless
+# optional FlashAttention is explicitly enabled and importable.
+# VIBEPOD_CUDA_DTYPE=bf16
+# VIBEPOD_ATTN_IMPL=sdpa
+# VIBEPOD_ENABLE_FLASH_ATTN=0
+
+# Debug/profiling. Keep disabled for benchmark timing; async CPU profiling
+# double-counts overlapped decode work.
+# VIBEPOD_PROFILE_GENERATION=0