The electric assist on e-Citibikes here in NYC diminishes with speed and cuts out completely at around 15 mph. Above that speed they're just a heavier normal Citibike. I know because I've enjoyed pushing these things to their limits.

There are many different types of e-bikes, and you're probably thinking of what are effective electric mopeds, with big batteries and engines and a throttle control. Those certainly go very fast. They're also not legal in many places, including here in NYC, and aren't necessarily what people mean when they're talking about e-bikes; that's usually referring to pedal-assist.