My niece as a toddler only had one speed: "damn the torpedoes, full speed ahead."
She had the ability to suddenly fall asleep, on occasion just leaning on something. One minute, she'd be a dynamo....the next minute just gone.
Of course, narcolepsy and other things were suspected and pursued. But in the end, it was medically decided that she lacked the bodily "governor" that told the body "SLOW DOWN, YOU'RE GETTING TIRED!", so she just kept going until the body just decided to shut down for a rest. The body knows best.
She outgrew it, thank goodness.