If You really believe the POTUS was under fire for a continuous 9 - 10+ Seconds, and Not a single member of the SS returned fire or so much as even left their car to try and locate/liquidate the shooter(s), then you are Indicting the SS. This 9 - 10+ seconds of active fire with Zero Proactive Response means the SS was either Totally Incompetent or Complicit. There's no 2 ways about it. Being under fire for a continuous 9 - 10+ seconds is an extremely Long Time. And then everybody just drives off? If you saw this in a B Movie you would say, "Never gonna happen".
We see here the reactions of some of the SS, after what most researchers believe was the 2nd shot, they look back towards where they heard shot/shots being fired from and I'm guessing they are aware the President has been hit.
For a start they weren't going to fire randomly into the crowd or at the buildings because that's just crazy.
Secondly, this was a sudden unique event, being fired upon while they were in moving cars, did their training include this scenario?
Thirdly, do they abandon the President and leave him potentially vulnerable to further attacks and leave local enforcement to locate the shooter?

In hindsight, they should have offered better protection, like riding constantly on the Limo.
Clint Hill was at different stages of the Parade riding on the Limo, but should have been there 100% of the time. And if IIRC JFK's side was partially ridden on as well.
And Greer ASAP should have put the pedal to the metal.

JohnM