Agreed they're very difficult to watch. I've had the same struggle.

Not only age but I am pretty sure overall health issues haven't helped.

Folks say he had migraine headaches his whole life, and back in 2019 unknowingly had what the medical community refers to as a "sentinel" headache.
His severe headache lasted roughly 12 days, then he ended up hospitalized for emergency surgery for a brain aneurysm.

They saved him, but when a person comes back from that kind of medical event... sometimes, they aren't exactly the same person.

This is something I have some limited experience with, that I wish I did not.
My Mom had a similar health problem but overall a worse experience due to lasting damage afterwards.
If my Mom made videos, I am pretty sure they would also be very difficult to watch.

I have never read or seen car or engine article by him.

I have read all his books, I watch his videos because it adds context to his writing and he puts in a little more than what the text conveys. Yes he talks too slow now and his speech seems deliberate, but the knowledge in those videos is worth the time. When guys like that are gone, they are gone forever, thank him for putting in the effort to make that knowledge basically forever on the internet.