Whisper is not installed. Run pip install openai-whisper and restart the server. FFmpeg must also be on PATH.

Model size guide (smaller = faster + lower quality):

• tiny — ~75 MB, very fast, ok for clear English
• base — ~150 MB, recommended starting point
• small — ~500 MB, good multilingual quality
• medium — ~1.5 GB, near-best quality, slow on CPU
• large — ~3 GB, best quality, very slow on CPU

Without a GPU, expect roughly 0.5×–2× real-time for tiny/base/small, and 5×–20× real-time for medium/large. A 10-minute audio file at medium on CPU can take 50+ minutes.