That sounds incredible. Do you have any videos of your table playing?
It's funny that you ask, because I brought up this possibility with my group as I have another friend who volunteered to set up sound, video and do the editing. However, I have two players who said absolutely not. So, no video - sorry.
I get that most of you are punctual, which my groups have always been too. And that you are all experienced, as has been almost every long-term group I've played with. And that your players decide prior to their turn, as do most players I know. But, how does it all pan out, say at 16th level, completing a threatening encounter for the PCs, at under an hour? What is your estimate in time for a dangerous 16 th level encounter?
Edit: A+ to fast being a relative term.
Here's what I have found - I cannot predict the time it will take. For example I threw a bunch of custom nasties at the PCs using some CR 17 creatures for the base. Employing some of the paladin's abilities and some spells - the opponent's were locked in place, under the effects of fear, and slowed. What I thought was going to be an awesome fight was quickly over. The whole thing was over in 10 minutes. I am also amazed at how fast they destroy combinations of creatures sometimes.
Now, I will admit, at the higher levels it is not uncommon for me to play some of their opponents at less than their full abilities. This is not intentional - I just plain forget sometimes. I could also be more inventive with spell lists. I learn and do better the next time, but it still happens.
So here's how I prep - I create ten or so encounters. I try to make sure at least half of the encounters have something interesting going on besides the fight - much, like the awesome 12th level fight you described, (which, I am totally stealing!). Moving pieces, magma, pits, verticality, waves of opponents, falling walls, etc. Usually combining a few.
Since it is important to end on time - I will sub in a more straight forward fight if we have twenty minutes or less to play. I never end a session mid-fight. Even though we play weekly, I do not trust my memory to keep track of where the fight was.
I consciously try not trump my players' characters' abilities. I also try not to pull them out of the fight - I avoid banishment as a spell for example. Although, I rudely used a Maze spell on the fighter last session. So, I absolutely could crank up the danger by going after weak spots or using more removal of PCs, but I always try to see things from their perspective. Is being banished as a player fun? Maybe every great once in a while, but not very often. I can tell you my fighter found being Mazed extremely frustrating and I'm not sure I would make the same choice again.
At least once a session, we have a fight that the players have real doubts about whether the going to pull through. They enjoy that style of play - a lot of folks do not.
I really admire Critical Role - they have an unbelievable campaign. The character interaction and the story telling is inspirational. And, my players would absolutely hate it.