On my drive home from work the Worth Way (a disused railway now shared use path) crosses the road. As I approached it a mountain-looking bike and rider appeared from it on my right and turned onto the road I was driving on a little in front of me. I've done the very same move myself when I cycle to work.
The rider had fairly good road sense, good number of shoulder checks, avoiding the larger potholes, that sort of thing. I eased off, as you do, as there was a bend coming up, poor sightlines, didn't want to crowd him. Then I realised he was going some and not pedaling. My speedo had him north of 35mph, nudging 40. I surmised he's on an electrically powered bike of the not legal sort. Either a homebrew conversion or one that's been hacked.
I can't put my finger on what annoyed me, other than I'm a law abiding kind of bloke. Over there > are the various road laws and in front of me was a bloke who broke them. It's not a "what might happen to him if he slides down the tarmac in his shorts" thing. I'll get the same result if I overcook a corner going at R17 on a pedal or pedal and gravity powered bike. He even slowed down when he entered the 30 limit and was riding sensibly, damn him.
I'd like to have got a closer look, but he got through the traffic and round a roundabout faster than I did.