I've had rental cars where it had an annoying lag and every start felt like a start from cold (ra-ta-ta-ta-voom), but in my current car there is no lag, by the

I've had rental cars where it had an annoying lag and every start felt like a start from cold (ra-ta-ta-ta-voom), but in my current car there is no lag, by the time I've lifted the brake the engine is ready to go. An auto-stop start is also much quieter than starting from zero. I'm not sure what it's doing - this car has an ISG (integrated starter-generator), I'm not sure if that's the difference.