I have played my current world for about a month and have not seen this issue until I just upgraded to 1.21.130.
There is a very noticeable lag when trying to turn a horse I am mounted on.
I noticed the same thing when on a boat as well.
The reason this is important is because if I am riding a relatively quick horse over a hill and there is a ravene or lava pool, I’m much more likely to die because of the half to full second delay in pressing the turn key vs actually turning.
I did not see the same delay when walking/running.
Did some additional testing:
I compared tapping the A and D keys repeatedly to slide left / right back and forth.
When I do that about every second when standing on the ground, the player shifts left or right almost immediately.
When doing it on a horse, it feels like my horse actually moves the opposite way due to the lag. I also tried W and S and the lag is just as bad.