His newer stuff has improved somewhat, he was working with a speech therapy group apparently in the last year or so.

It's still not perfect but much better than it used to be.

He has closed captioning on all his videos to help viewers.

I think it's fine if you have an issue, but just so others no I have never had any issues understanding him, and he has gotten a lot better.

Issac Arthur is a treasure, I wish high schoolers could take a semester of him.

Play it faster. 2X or more.

At first, I just thought it was some kind of exotic accent but I have had no problem understanding him and english is not my first language. In fact, I find his speech quite enjoyable.

So to anyone unfamiliar with his work, do not let this stop you from watching, it's awesome if you are in to this kind of stuff.