I clocked some of that last clip in Transcribe!, Roy was hitting about 21-22 notes per second when tremolo picking. Very fast, but not unheard of.
I think his picks must be made of a very hard material. When making initial contact with a string, a very hard pick will will effectively sound a note determined by the picking position (similar to a slide touching the string or Bumblefoot’s thimble-tapping technique), before it pushes through the string and sounds the intended note.
As most people pick somewhere between the fretboard and bridge pickup, the resulting non-musical pitch is very audible when playing on the bridge pickup.
I can’t perceive it with a nylon pick unless I’m picking a string that’s damped with the left hand. I perceive it more clearly with something like ultex, but it’s still in the realm of what I would consider “attack” when note are being fretted. With lexan (the material stubby picks are made from), the non-musical contact pitch is very noticeable to me on every picked note, and I don’t like it.
I think some people call this “chirp.”
Roy seem to get around the shrill sound of the chirp to some degree by picking at the bridge itself, but this results in a very thin tone, and it means he can’t effectively damp strings with his picking hand. I suspect that this is why he needs the ribbons on the neck for string damping, and why he favors the synth pickup.