You are right, it might only change FOV. I did not measure speed, and I'm not sure if I know how.
Yes, this happens in 1.15.2 also. No, that issue is not what mine is. When I push CTRL and W, you are supposed to sprint in the minecart to go faster. The bug causes the sprint to be "lost", causing you to stop sprinting in the minecart. If CTRL is held down, it will cause a pulsing where the CTRL causes the minecart to sprint, but the bug stops it. It causes a jerky FOV and speed.
I have the same thing in snapshot 19w40a
That is not the same problem. The "duplicate" describes an issue with the wrong FOV being present. My issue describes a stuttering and inconsistent FOV WHILE in the minecart, not after getting out.