I have seen studies saying that EEGs have shown abnormal readings following concussion, and seen another saying that these abnormal readings are not uniform or specific enough to concussion to be diagnostic (ie they can't diagnose you with anything purely on the basis of these readings). There are different sorts of brain waves based on speed, frequency or whatever, and these can apparently be different following concussion. I don't think that slower brain waves necessarily equate to your brain running more slowly, I think it's more complicated than that, though I may be wrong.

Sorry it was a while ago so I don't remember any more about these studies, so that's probably not very helpful!

fMRI studies hve shown that different areas of the brain light up when people engage in a cognitive task following mild TBI than in a control group, even if performance levels are the same: the assumption was that the brain is compensating for lowered efficiency by recruiting different areas to achieve the same result. This could be reflected in EEG readings somehow, so that might be the source of any unusual readings. It also seems to me to be a good reason why we get tired - the brain is working harder just to keep up.

These sorts of active imaging techniques are something I'd like to learn a lot more about.