I'm watching this thread since we are going over Xmas this year, our first time during the holidays. But - we were at WDW last year for Thanksgiving.
This was us exactly. We had no idea that Christmas Day would be so insane at magic kingdom. We could not move in the crowds. It was really awful. This sounds pretty snobby but it was also not a very sophisticated crowd. I am not sure why. Never again.
This was us on the Wednesday before Thanksgiving, 2018 - we could not move on Main St. The app said Small World was a 25 minute wait, and it turned out to be an 80-90 minute wait. That day we did our 2 FPs, rode the carousel after waiting 20-30 minutes, stood in the awful Small World wait with our almost-3yo who wanted to be held most of the time, and headed out. It was projected to be a 10/10 on Touring plans and it felt like it.
We did not go to HS on that trip, but both EP and AK were MUCH more pleasant experiences that week.
Re: "not a very sophisticated crowd" - for Thanksgiving I saw several large groups wearing family reunion t-shirts and "First Visit" buttons. I could see there being many more WDW newbies on a first-ever or "one and done" family trip, and that might explain how it could be especially difficult to move, and especially in MK.
.we have used this technique and though I have no "science" to confirm it, I do think going left is moving with a smaller crowd though the later you go in the less impact this strategy will have.
I'm a lefty - I think it has to do with right/left brain and which side is dominant. I have read quasi-scientific studies that more righties have a right-hand-side bias. The left side line often is less crowded, but people don't always "see" it. If you have 2 side-by-side escalators in the airport, for example (or side by side TSA screening lines), the leftmost line usually has fewer people in it.
We are not doing parks on Christmas Eve since that's when we are going to Mass and preparing our dinner , but we are going to AK on Christmas Day since we will be at AK at that time. I wonder how bad it gets at AK on Christmas Day!
Last Thanksgiving we stayed at BLT and were there Tuesday - Sunday. Tuesday, Wednesday and Saturday (?) were our MK days and they were wall to wall people. Thursday (Thanksgiving) we went to AK and it wasn't bad at all. Friday we were at EP and we spent most of our time exploring the countries and seeing the various shows - we managed to time it just right to hit a bunch of shows walking counterclockwise. It was very pleasant compared to MK.