Biking is not always more efficient, as this quora answer points out: https://www.quora.com/Is-it-more-energy-efficient-to-walk-run-or-bike-up-a-hill/answer/Raj-Kumar-855
Regarding muscles, as an (former) cycling enthusiast, cycling uses a lot fewer muscles than running. The muscles used for cycling are some of the strongest in the body (hamstring, quadriceps). Compared to running which uses almost the whole body, including core muscles for stability, this is a downside for cycling, because it may not be as natural as running. But I wouldn't say you end up tiring weaker muscles.
2. The speed at which balancing becomes effortless is roughly walking speed. It is actually difficult to ride that slowly (depending on the bike, load, windspeed etc).
I wonder if there's a simple mechanical fix (apart from getting off your bike and walking it uphill)
Incidentally, there is one such fix. It's called gears. I reckon (mostly based on personal experience, haven't done any calculations), that even uphill, taking the weight of the bike into account, cycling is more efficient than walking at the same speed. (But probably not 3x more efficient, as it's the case on level ground).
currently the most advanced form of communication to have evolved, human language, is still very low bandwidth
Language is just one of the means of communication available to human beings. Its seemingly low bandwidth is due to the fact that the neocortex, the part of the brain responsible for processing language, logic, visual reasoning etc, is itself rather low bandwidth/low speed, so language itself isn't the bottleneck. (The bottleneck itself is mainly due to the more recent evolution of this neocortex, and the fact that these processes were not that vital to human survival until very recently on an evolutionary timescale. Also that these logical processes take up a lot more energy than the some of the other processes happening in the brain.)
But human nervous systems do have much higher bandwidth communication channels. We share them with the other mammals. It's the limbic system, that based on the tone of another person's voice, his posture, his facial muscles and a lot of other indicators, leads to a split second reaction, a gut feeling, an instinct about another person, his standing/status in the herd, etc being formed.
Going a level lower, there is an even faster, more primitive system, the basal ganglia, or the so called reptilian brain, this is where sensory input is first processed. For instance, in a split second you can tell if some set of input represents a dangerous situation, leading to a fear response.
In complex vertebrates it is the amygdala that first processes these sensory inputs, matches them against a fast type of emotional memory, and triggers the appropriate response through connections to the hypothalamus (for instance, triggering the flight-flight-freeze response).
In any case, I think the question itself is a bit naive on the brain architecture side from an evolutionary perspective. The nervous systems of these mammals do process and communicate a great deal of information (of course, this being an evolutionary process, there are things other than the brain and communication competing for energy). In the case of humans, most of that communication is not conscious. The part that is conscious is just a very recent addition, and was not that vital until very recently.
I recommend the book "Why Zebras Don't Get Ulcers" by neuroendocrinologist Robert Sapolsky, in which the exact mechanisms of the stress-response are being described, alongside with a long numbers of experiments and a huge body of evidence pointing towards how harmful chronic stress really is, how it is at the root of an astounding number of pathologies from the metabolic syndrome, ulcers, gastritis, to cardiovascular diseases, to autoimmune diseases, to mental illnesses like depression.
The recurring theme of the book is how the stress response is adaptive in the acute, short term form, but becomes maladaptive in the chronic from (particularly through elevated levels of glucocorticoids).
So, what you describe fits quite well with my own experiences.
On a meta note, this LW-specific habit of introducing new names/labels for things that already have clear, well-established terminology is particularly bothering. Especially when the name is completely unrelated to the concept (i.e. lotus leaves). (OTOH, the social forces that drive this habit are easy to intuit).
(On topic, like most if not all of us have dealt with the intricacies of the dopaminergic system, perhaps I will at one point write my experiences on my blog).
Survey has been taken.