A big problem I see on an ongoing basis is the confusion between time spent working and work produced. After ~10 years in this field, I feel like a pitcher who loses their fastball sometimes, and have learned when to sense that and pull myself from the game (stop working). I feel for people who don't see or sense their performance deteriorating in a way that just calling it a day and coming back with fresh eyes could improve.
Should you just plow through sometimes? Definitely. But if I find myself needing to consider that for more than about a week a quarter, someone probably planned poorly or is taking advantage of the fact that I care about completing my work.
There are other comments about work vs personal/skills development - the latter I do not consider work, personally, even if they're inter-related at times. I have actually noticed a big difference between my previous employer and current employer. I had enough slack at my previous employer to pack in quite a bit of high value personal/skills development after I finished my work and on weekends. Ironically, its this slack that gave me the time to learn skills that I was eventually hired for at my current employer - however, I have none of this mental bandwidth or time at my current employer. I can't figure out how much of that is pandemic-related, new child related (had a baby while at the new employer), or new employer culture/balance issues with how I work best. I guess it's just hard to track the true cause when the world goes into a blender while one's life changes significantly too.