The MIDI preview render (~2 s for a 5 min song) and auto-align ran synchronously on the
server's event loop, and each preview was a 12.4 MB WAV. Renders now run as 4 parallel time
chunks (2 s pre-roll, 10 ms cross-fade) on worker threads, auto-align on its own worker, and
previews are mono Ogg Opus 48 kb/s via ffmpeg (1.5 MB, same timing as the WAV in Chrome).
?format=wav keeps the PCM output. Swimming: render 2.0 s -> 0.65 s + 0.6 s encode; event loop
blocked 2.0 s -> <0.1 s per render.
Co-Authored-By: Claude Opus 5.5 <[email protected]>
