How important is getting enough glucose for maximal brain performance / cognition?
In short: skipping a meal lowers blood glucose levels, which can reduce cognitive performance, for example, memory.

Let's say that between meals you are in fasting state for 8 hours during which you do some desk work and some moderate exercise for 1.5 hours. With such level of physical activity, your last meal and your glycogen stores should provide enough glucose for all your body needs, including the brain. So, during this time, you do not need any ketones or lactate as additional source of energy. According to Medical Hypotheses, 2014:


  Estimated amount of glycogen in the human organism is about 200–500 g,
  and in certain specific conditions could reach up to 1.0 kg, which
  seems to be optimal for human organism. This amount is definitely
  sufficient for keeping optimal concentration of glucose in the blood
  during usual fasting periods not exceeding 24 h (between meals, during
  sleep, in case of exercise, etc.)


Skipping a meal can lower your blood glucose levels. According to WebMD:


  Normal blood sugar levels are less than 100 mg/dL after not eating
  (fasting) for at least eight hours. And they're less than 140 mg/dL
  two hours after eating.
  
  During the day, levels tend to be at their lowest just before meals.
  For most people without diabetes, blood sugar levels before meals
  hover around 70 to 80 mg/dL. For some people, 60 is normal; for
  others, 90 is the norm.


Blood glucose fluctuations related to meals:



Image source: Wikipedia, Creative Commons License

In some studies (AJCN, 1998), skipping breakfast was associated with lower blood glucose levels and worse memory test but not intelligence test results:


  This article compares the findings of three studies that explored the
  role of increased blood glucose in improving memory function for
  subjects who ate breakfast. An initial improvement in memory function
  for these subjects was found to correlate with blood glucose
  concentrations. In subsequent studies, morning fasting was found to
  adversely affect the ability to recall a word list and a story read
  aloud, as well as recall items while counting backwards. Failure to
  eat breakfast did not affect performance on an intelligence test.