[Fat Charlie]

Looking at the glide distance, aero drag is stronger at higher speeds so you'll lose speed faster at higher speeds, which cuts its utility. Looking at fuel consumption while gliding, you'll have more injector pulses per foot travelled at lower speeds, so the benefit of engine off is magnified there.

But I wouldn't say it's only useful at lower speeds, just that at higher speeds you need a longer glide for it to make sense, generally with some sort of a downhill to help extend the glide while minimizing speed loss. It doesn't take much of a downhill to help- one of my morning EOCs is on a stretch of the highway that I hadn't realized was an incline until I started hypermiling.

But yeah, higher speed reduces your ability to save fuel.