It's a bicycle....
I think it's rather too easy to take one's eye off the ball here. The idea is not to allow you to go faster than a fit cyclist on a road bike could manage, but to take some of the pain out of getting up hills and to assist - the operative word - the less fit and the heavier among us to use a bicycle effectively. I suppose 15 mph is considered a reasonable average speed for a cyclist, and to fit in with that, the legislation regarding motor power and when it's available and when it's not has come along.
If you want to go faster than 15mph you can - but only under your own steam. There's nothing unreasonable about that. Of course, you can go a LOT faster than that, and under power - but then you don't benefit from riding a 'bicycle' because it becomes a powered motor vehicle, with insurance, licence, and all the other paraphernalia put there to protect us from ourselves and each other.
All the arguments along the lines of 'If only we could have a little more power, and/or go a few mph faster' tend to lose sight of the fact that it's an assisted bicycle. In my long motorcycling career I've heard the same argument put forward about mopeds - 30mph and weedy acceleration make a moped rider vulnerable, and it's a much better idea to have more power available to 'get out of trouble'. Or, of course, pass a proper test and get a proper motorbike. The power and the speed you seek are all available, and if you feel the need then maybe an e-bike is not what you should be using.
My limited experience as a new e-biker is that it's fun in its own right, and the perceived limitations just mean that it's different. It's easy to ride the Wisper round here at 20mph average, and London area traffic doesn't usually manage to go much faster than that - often a good deal slower. Also, if I can use the pavement and take it on the train, I can go anywhere at all. It gets me out and pedalling, which I know I wouldn't do without the 'assistance', and there's some slight satisfaction in the fact that the Wisper lets me cheat a little on the 15mph limit. Not enough to attract unwelcome attention from the police, though.
Rog.