Speed of Brain Waves
Unfortunately the term "wave" is ambiguous in neuroscience. What you are referring to, alpha waves, only means that neuronal activity tends to oscillate at about 10Hz. Neural oscillations are a widespread phenomena occurring in all brain areas. But it doesn't necessarily mean these waves travels. There are examples of traveling waves, whose speed can vary a lot (from $10^{-1}$ to $10^{-5}\ m.s^{-1}$ depending on the studies). Oscillations across brain areas tends to be synchronized during a task (so one could argue, infinite speed), and within area activity tends to hold a specific phase relationship which varies spatially (as if a wave was "frozen", in which case its speed would be 0). Traveling waves and neural synchrony/coherence might be different phenomena implementing different functions. It is still unclear what brain oscillations are for, or even if they have any purpose at all. Below are some reviews on these topics.



Ermentrout, G.B. and Kleinfeld, D. (2001) Traveling electrical waves in cortex: insights from phase dynamics and speculation on a computational role. Neuron 29, 33–44

Sato, T. K., Nauhaus, I., &amp; Carandini, M. (2012). Traveling waves in visual cortex. Neuron, 75(2), 218-229.

Engel, A. K., Fries, P., &amp; Singer, W. (2001). Dynamic predictions: oscillations and synchrony in top–down processing. Nature Reviews Neuroscience, 2(10), 704-716.

Fries, P. (2005). A mechanism for cognitive dynamics: neuronal communication through neuronal coherence. Trends in cognitive sciences, 9(10), 474-480.