I have several variations upon a theme with this.
I have the heavy legs, heavy everything variant, most noticeable these days is heavy head.
I also have "enough" energy but it won't go anywhere, I seem to have the energy but i can't drive my limbs any faster with it.
I also have a "this light thing is too heavy to move" variant - first noticed when trying to lift an essentially empty box (it had a lampshade in) from waist height to put it on a shelf in the early 90s. I could feel it wasn't heavy, but could I lift it? Nope. I seem to remember getting quite annoyed about it that first time lol.
The effect is quite pronounced and is at least a 20 fold decrease in apparent strength*, for anything from a few seconds to ages. I seem to have noticed a correlation between this type and needing to rest to reduce or even avoid PEM. Paying attention and noticing it rather than just overpowering it seems to lead to less PEM than just ignoring it, as I always have done before. Probably not explaining it well.**
*edit - I've measured it, with weights lol
**edit - I didn't. What I mean is I noticed that the day before a crash I often experienced this sudden loss of strength, sometimes only for a few seconds, just little things like struggling to open a cupboard door, nearly dropping, or dropping, a plate because I no longer have the grip strength to hold on to it etc. When I notice it I am now backing off, and resting for the rest of the day, and so far I've prevented my existing crash from getting any worse, which given how bad I've been, and how much I still have to do on a routine basis, is pretty remarkable

It's a theory, sudden fade is a warning to back off, so far it seems to pan out, for why, no idea.
Or - many, many less words to say the same thing.