You can slow the kite (turning) a bit down with a shorter bar, yes.
But it does not actually make the kite slower by the same amount you shorten the bar.
Difficult to explain, but one could say, that if you hold your hands at the bar ends (you dont normally, I know), then you should pull with exactly the same amount and force for a given kite, no matter if a short or long bar
So here it does not slow the kite down
Because we grab the bar further away from the bar ends, it still somehow feels like the kite is slowed down a tad yes, but not that much really.
So unless you turn the kite with a lot of bar angle, one could also claim that short or long bar does not change the turning speed (feel) of the kite
It IS very tricky in fact, because of this two-faced issue.
Somehow small fast kites are still very (too much for many) live and twitchy for freestyle (but fine in waves and for high jumps), even with a short bar.