This came up in the recent thread about giraffes recently (linked below). I also posted a video of my penguin (a geared 19" trials wheel, 2:1 ratio with 150mm cranks, similar to a 36er with 125 cranks) as well. There is no off-the-shelf solution. If you want the speed of a 36 in a more compact package, a schlumpf hub is the way to go if you can afford it. It’s very close to a 1:1.5 ratio, so a 24" wheel is geared like a 36. They are more difficult to ride than a typical ungeared big wheel, but not unreasonably so. If you want a cheap, portable, durable solution, a 29" wheel with short cranks is a great option. My penguin is slower than my 36er, but it will cruise above 10 mph.
A schlumpf only has a 1:1 and a 1:1.5, so you don’t get a ton of value out of a smaller wheel, but someone on the forums slapped on in within this year. That thread shouldn’t be too hard to hunt down. It basically turns a 20" into a 29er.
The third option is a jackshaft, which I haven’t seen done on such a small wheel. (you would have to mount the cranks above the hub, where on most jackshafts, they are mounted below to lower the center of gravity I would guess.) Unicycle.com sold a 26" jackshaft called a Huni-Rex I believe, but the cranks were so low to the ground that pedal strikes while turning was a problem. You can still find them on sale used every once in a while.
I have found that despite what wheel you’re on, your cruising speed will be limited by how brazen you are, and the terrain you ride. For smooth roads, and a geared 36er (likely the fastest solution outside the jackshaft with large gearing), you can get it up above 20 mph, but I was never able to warrant keeping that speed for too long. I generally kept it around 16-18 on smooth bike path. TL;DR: never as fast as a real road bike, but enough to go on long rides or get you around town.
There’s at least one CVT uni with an electric assist that Justin created, but I’m willing to wager nobody is going to consider that as an actual commuting solution.
here’s a few links that you may find helpful:
edit: for clarification, the jackshaft in the first link actually has the pedals in the same location they normally would be and uses a different mechanism than the huni-rex, which, only having one chain on each side of the wheel, forces you to move the cranks. See the link for a more detailed explanation of the jackshaft. The huni-rex is pretty self-explanatory from pictures.