I can't think of very many things that affect performance in big ways. An explicit "I'm going to do a lot of things to the DOM here, please hold the reflows until I'm done, I understand that some of the properties may go out of date in the meantime" might be nice. (Perhaps it's a death-of-a-thousand-cuts.)

The API itself is a bit klunky, but the rough edges are easy to paper over with any number of libraries, and is hardly the world's first klunky-but-functional API to be so papered over by libraries that add no significant slowdown. (Note I'm not talking about "react" here, but just things that make the DOM API a bit less klunky. Thin little wrappers that mostly get JIT'd out.) Native XPath-like integration into JS would be nice, but honestly, that's going to be a net slowdown because people will do lots of slow, easy things rather than write the more tedious, but faster, DOM manipulation.

But just being klunky doesn't actually make it slow.