The timing is always exactly the same under 9u/t or 600u/s Vertically. Just before you hit the ground or psuedoground in the case of speedbounces.
Any vertical velocity higher than 600u/s it becomes a lot more like a bounce just with a much much wider window where the player can fall into.
In both of my 3500u/s speedshot videos I was travelling 1750ish u/s horizontally and obviously I managed to hit a speedshot, prefire or not, just fine. But I would say that about 2000u/s horizontally is pretty much the max of what's possible before the amount of vertical height you get after the speedshot to become just useless.
There is no hard number on the horizontal currently mainly because no one's looked for it and it really doesn't matter outside of TAS, just tap s when needed.
As far as aim angle I have no idea for me ~45 degrees down works fine in between ss and flicking down to 70-80+ on the actual shot.
Because people always want a link instead of googling