No it wasn't.
But rethinking. I think that the high speed behaviour is as to be expected. With the low speed unit removed, the lever preventing the firing of the second curtain is activated. That explains normal behaviour when I activate it by hand.
I think now that I did not pay enough attention to the position of the cam operating this level in replacing the low speed unit.
What remains then is that the first curtain does not completely return to the starting position. Perhaps I should tighten the spring with half to one turn?