Did some fresher testing of my own as well today and noticed a few things:
Frame Cap OFFThis is me just running VC:MP like anyone else would
As you can see, my system is holding 60 FPS but my frame time (the green line) is all over the place. This is normal with any game you play to be honest so not a point to fuss on, just note the amplitude of those spikes. I'd spike anywhere from about 14 to 20 ms, averaging the 60 FPS.
When in-game, you can see and feel the jitteriness to it, or at least I can. It's okay if you've never experienced jitter free gameplay before but its noticeable once you have.
Different map but take my word for it, same result really; this is how it looks when you disable the border. Frame capping is still off, and as you can see, the frame time looks much more relaxed than the first one. This graph is a typical VC:MP vanilla experience, but it still can be improved upon.
Frame Cap ONThis is with frame cap on in the same game of the first image. As you can see, it smooths out all that jitter and provides a much cleaner experience. Do note however those momentary spikes. I wasn't able to see it but I can say that the game didn't feel like my specs were overkill, more felt like when you run a game max detail and all is good but not perfect. It just felt right, not pristine.
Then you have the fourth experience, frame cap on with border disabled. Didn't bother to take a pic cuz its nearly the same as the 3rd image you see, the only difference was that those momentary spikes you see in the third image were significantly reduced. I didn't see a difference but I definitely felt a difference and would attribute it to less input lag (but I have no real way to measure that atm). I will not lie, I definitely noticed it. Border off was what felt like my system was overkill and pristine as it should.
Now I might know how to cap my framerate to deliver a smooth experience but the typical VC:MP player just hops in and does his thing. This is how my end respond but other systems might be doing much worse than what I'm capable of so I see the point.
Again I ask to try KoTH with border disabled and tell me if you get the same experience like I'm expressing, but from what you showed of the RTV video, I would say that an optimization that could be made in general of GUIs is
not to have translucent designs and avoid sprites if you can, but if you have to use it, use very minimal amounts.
Another thing that you have to and must avoid is font resizing. We were going to animate the countdown to 1 in KoTH, like a pulse effect, but that brought every system to its knees, the VC:MP code around it is not good. It made wep selection lag severely so we had to scrap that idea