@SOC and everyone in general: just chiming in because I'm really pleased with the results I've had in alleviating PEM in the last 1.5 or so years since I hit PR. I had gotten to where I pretty much couldn't exercise at all beyond slow-walking my dogs. This forum is both a goldmine and a godsend, for sure.
Alleviation of PEM began when I got methylation going with Freddd's protocol. I noticed a significant boost in energy when I added adb12 and LCF to the mfolate and mB12 (which I tried first). The energy boost was also accompanied by jitters, anxiety, and mild insomnia, plus a few flu-like crashes. It was not a comfy stable energy but still was better than my normal fatigue + ennui. The LCF/adb12 did not completely alleviate PEM but it was after these that I began to tolerate exercise better, and when PEM came as it always did, it was different. Shorter and not as bad. After a short time I began to feel well enough to attempt a bout of aerobic/strenuous exercise 2-3 weeks after the previous attempt, instead of having to wait two months or longer between (the almost heartbreaking) attempts. I also began to experience an actual desire to move my body instead of exercise being something I knew I "should" do for my own good, but still had to drag myself to it. The longer I stuck with the Deadlock the better it all seemed to get.
High dose ubiquinol added the next dimension to energy/exercise tolerance/PEM diminishment. I started taking it after some dental surgery to help heal my mouth, and unexpectedly discovered a great little boost. I had to take at least 600mg/day of ubiquinol to get that boost, though. Under that dose and it didn't seem to help with energy although my mouth and gums seemed to like it. Interestingly, before this I had written off CoQ10 as something that didn't really work for me, although I always took a couple hundred mg/day on faith, since it was considered by all to be a crucial antioxidant. But I had experimented with high-dose several times in the past and got zero effect from it. Then suddenly, after I'd done some work on methylation and mitochondria, it worked for me like I'd read it was supposed to work. So I think I had to correct some methylation and possibly mitochondrial problems before I could respond to it. As
@SOC noted, it's not a cure-all, but if you can get all the right things in place so it does work, CoQ10 is a great supplement.
I'm currently taking the MitoQ brand of CoQ10 and liking it a lot. I think it is all that, and if you respond to ubiquinol it's an even better addition to a regimen targeting energy, activity tolerance, and PEM. I'm taking the recommended 10mg/day dose but am going to experiment with half that since I think it makes me a little speedy and I'm not sure I need the whole 10mg. While pricey, MitoQ seems to be equivalent to or maybe a little cheaper than high-dose ubiquinol, simply because of how much ubiquinol you have to take to achieve a noticeable boost. When you compare the two different forms and how much you have to take, the price is a lot more equal than if you look at price alone.
Thiamine + choline seem to fill in another big piece of the exercise/activity tolerance puzzle, but please don't ask me how or why. I still haven't figured it out. My brain still isn't firing on all cylinders, and when something seems to work for me I'm not always sufficiently bothered to figure out why unless I have time. I just keep doing it and don't worry about it.
Seriously, I've read up a bit on it and think I have an inkling but am thinking too slowly to have the time to piece it all together. I just know that boosting these two things seemed to add a whole other crucial dimension to my exercise tolerance. Worth noting is that the initial boost was followed by a severe crash that seemed to be a gastrointestinal flu, but after I recovered from that flu my stable (i.e., long-term) activity tolerance improved remarkably and PEM became very short to almost non-existent. I still have to be careful so I don't run myself down, but I'm now exercising regularly several times per week and getting stronger every day.
The other final thing I think was a factor in alleviating PEM for me was fixing my gut. A long gnarly journey, but worth it. Not there yet but good progress.
Just my $0.02. Sayin' because most of us are on similar journeys, just at different paces.