Benchmarking CrispASR

July 17, 2026 · View on GitHub

How to measure CrispASR's speed fairly — whether you're tuning it, comparing it against another engine, or publishing numbers.

The one rule: measure transcribe time, not cold start. CrispASR ships as a single binary that loads a multi-GB GGUF and uploads weights to the GPU on every process start. Timing crispasr -m model.gguf -f clip.wav end-to-end with a stopwatch measures process start + model load + GPU upload + audio decode + transcription. Most engines you'd compare against are benchmarked in-process with the model loaded once and the first (cold) rep discarded — so an end-to-end subprocess timing isn't the same quantity, and the gap it shows is mostly load time.

Every method below isolates the transcription.

Quickest fair number: read the stderr timing line

Both the CLI and the server already print a load-excluded transcription time:

crispasr: transcribed 11.0s audio in 0.42s (26.2x realtime)

The timer starts after model init, audio decode, and VAD/slicing (crispasr_run.cpp), so this line is transcription only. The server prints the same shape (crispasr-server: transcribed ... (Nx realtime)) from its own elapsed_s, measured around the transcribe call.

If you script around the CLI, parse this line — don't wrap the process in time.

Method 1 — server mode (resident model)

Best match for how a service actually runs, and for engines benchmarked in "server mode". The model is loaded once at startup and reused for every request.

# Start once; the model stays resident.
crispasr --server --backend parakeet -m parakeet-ctc-0.6b-q8_0.gguf \
    --host 127.0.0.1 --port 8080

# Then send N requests. Discard the first (warms JIT/pipeline caches).
curl -s -F file=@clip.wav -F response_format=json \
    http://127.0.0.1:8080/v1/audio/transcriptions

The server's stderr timing line per request is the transcribe time. See server.md for the full endpoint/field reference.

Method 2 — in-process (the apples-to-apples path)

Load the library once, transcribe many times. Use the Python Session API (python/crispasr) or ctypes against libcrispasr.{dylib,so,dll}.

This is what tools/benchmark_asr_engines.py --crispasr-call ctypes does, and it's the path to prefer for cross-engine comparisons — the engine is loaded once and reused across runs, matching how the other side is usually measured.

Method 3 — CLI with warm reps

Acceptable if you parse the stderr timing line (above) rather than wall-clock the process. You still pay process start + model load per call, so only the parsed line is meaningful. For a load-amortised comparison, prefer method 1 or 2.

Proof-of-work — an RTF is a lie until you prove the work happened

rtf = audio_duration / walltime with no output check turns a crash into a record-breaking result. A wrong-backend load failure that exits in 0.5 s once minted a fake "102× realtime". Defenses, in order of value:

Any non-zero exit or empty transcript is a FAIL, never a timing.
Scale check. Benchmark a clip that is a known multiple of a short one (e.g. 55 s = 11 s × 5) and assert the transcript word count scales ~5×. The tell for a fake win is that the "55 s" run takes the same time as the "11 s" run — fixed time ⇒ it isn't processing the audio.
Warm up per shape, take a median of ≥3, and print absolute ms next to the RTF. A 0.06 s denominator magnifies noise; an un-warmed GPU shape fakes a collapse.
Discard the first (cold) run.

Measure both arms under identical load

A noisy box fabricates wins. GPU timing on a loaded 16 GB M1 swings ±20 % (the same graph measured 127 ms/frame quiet vs 264 ms/frame loaded). A quiet-CPU-vs-loaded-GPU comparison once invented a "2× CPU win" that a clean back-to-back run reversed.

Check sysctl vm.loadavg (macOS) / uptime before every timing run.
Run both arms back-to-back, alternating order.
Run each config as a separate process — a second crispasr spawned immediately after the first in the same shell can exit at 0 s on a GPU/resource-release race.
For a verdict you'll publish, prefer a quiet dedicated box.

Report the configuration

An RTF without these is not reproducible: backend, quant, device/backend (Metal/CUDA/CPU+BLAS), thread count, warm vs cold, load-excluded or not, clip length, and absolute ms. Quant and BLAS quality dominate: the same parakeet decode measured 955 ms on one CPU (OpenBLAS) vs ~60 ms on M1 (Accelerate) — ~16× from BLAS alone, same code.

Phase timing / profiling

Per-backend stage timers are env-gated:

Env	Effect
`CRISPASR_VERBOSE=1` (or `--verbose`)	Turns on every backend's debug/bench vars at once
`CRISPASR_<BACKEND>_BENCH=1`	Per-backend stage timings (e.g. `CRISPASR_CANARY_BENCH=1`)
`CRISPASR_METAL_PROFILE=1`	Metal host-encode vs GPU split
`CRISPASR_FC_PROFILE=1`	Per-node profile for the FastConformer family (parakeet/canary/…)

Existing harnesses:

tools/benchmark_phases.sh — per-phase benchmark across ASR backends, short (11 s) and long (55 s) audio.
tools/benchmark_asr_engines.py — head-to-head vs onnx-asr on the same Parakeet model; quant × mode matrix, JSON output, warmup handling. See ../tools/benchmark_asr_engines.README.md.
PERFORMANCE.md — current published numbers. Read it before benchmarking; it is the source of truth for engine/quant status.