How do you know if it's full speed?

What if it's 90% of full speed? 75%? 10%?

And how many times have you seen a player crash into a defender at "full speed" and the defender isn't displaced? Can't say that I've ever seen it.

Full speed or not, the OP's situation was that he "slams" into the defender with LGP. I guess I agree with Jimgolf.

I've seen kids drive the lane and slam into the defender, and unless he's coming from half court building speed, this "slam" isn't normally that hard. Usually, it looks a lot worse for A1 because of the complete and sudden stop.

I'll reiterate. If the kid comes in like a cannon ball, call the foul. If the defender gets hit that hard, though, there's going to be some displacement.