Okay, this is to follow up on my previous post. It seems that the big problem is actually in the Reduce Judder algorithm itself.
I tested the same scene again and again under different circumstances to try narrow down the problem, but finally nailed it. When playing a 2.35:1 movie (meaning - has black bars while playing on the TV), the Reduce Judder algorithm fails on many fast action shots (always the same shots, it's not random) and it becomes full of judder for those shots, distracting while watching the entire movie as it's smooth 90% of the time then juddery every now and then.
And here's the kicker and don't even ask how I found out - if you're playing the exact same scene but there's something on the black bars (like player controls, clock, video file information, etc) - then it plays flawlessly and judder-free, including on the problematic scenes.
I tested this with the subway scene of John Wick 2. There's a subway arriving to the station. Completely judders UNLESS the player controls are on the bottom of the screen (because you were seeking or clicked 'info') - in which case it becomes magically smooth like the rest of the content.
I made triple sure that my player itself doesn't change anything in the content when there's something on the black bars, and tested multiple players. It's just the way 'reduce judder' processes the scene when it sees there's only content in the middle of the screen as opposed to when it thinks you're playing something full screen.
I hope this somehow helps and there's something to be done about this.