I can relate. Recently I've started working for big company and they have quite complicated and over engineered microservice architecture - the product I'm working on.
What I found is that I'm slowest dev in the team. I can do my work well but I'm spending time improving things that are not important in the scope of the ticket I'm doing.
I've tried to convince myself that this is because I'm trying to address issues with infrastructure but recently I started thinking "maybe that's because I'm getting old, and my performance is declining".
But on the other hand I found that I'm still "clever" and fast and bright if it comes to things that got my interest.
In my case it seems like some kind of middle age crysis. I'm just simply getting bored with programming, I've seen all of those issues before, now it's not different except scale, and I just feel like my effort invested in improving will be like drop in the ocean. Where back in my 20th I would flip everything upside down until I would succeed and make a difference.
What I'm trying to say is: Maybe you just getting bored of those problems...
and thinking about mental decline is natural thing in our age.