I was thinking of starting a poll about the time delay, because I wondered if there was a subset of patients who shared the ~24 hr delay, but this thread seems to answer that quite clearly. My hypothesis is that the 24 hr delay is due to IFN-g released by t-cells that were cleaning up the micro-tears in muscles caused by physical exertion. The time delay is right for that. The different responses to using muscles in ways that do or don't cause significant muscle damage concurs with that. I know that my symptoms increase with activities that would increase IFN-g, and viral infections which do the same, and with increases in tryptophan transport into my brain.
A quick check shows some research papers linking CFS and IFN-g. I was going to look for papers detailing the time delay between muscle damage and IFN-g rise, but I drove to town yesterday and the PEM is starting, so that's out for another day or so.
For people who have much shorter delays, I expect that's a different process. IL-6, for example, rises quickly with physical activity.