I've been burned out twice before. This cognitive sluggishness is similar, but not identical. Lately there's nothing particular going on at work that has had a consuming or stressful effect on me, but that doesn't entirely rule it out.
Make sure you don't have https://en.wikipedia.org/wiki/Boreout as well, which I've actually experienced (it led to a lot of resentment and then led to classic burnout).
It's always worth ruling out these things because they are more common than the really scary physical issues. That said, always worth getting physically checked out if concerned etc.
I swear that Bore-out is an under-discussed topic with remote work. The pursuit of WLB can easily turn into a do nothing job. A significant portion of the population want to describe their job as "exciting" or "fast".
