The resource limits are not loading.

Left panel will not collapse - but it moves center over to the left:

Newsletter signup is messed up:

In the API Explore there is a paper clip thingy. When you click on it, the list randomly goes to some other API category.

Yeah, I'll do that next time I hit it and have a real repro to provide context.

That specific performance issue is a client-side concern. The new and old UI apps are the same code in that case. The cause of that performance issue is the syntax highlighter on the API response payload. The tipping point of when that performance becomes a problem is highly dependent on your computer's specs. There is a limit in place that disables the syntax highlighter above a certain payload size, but we can consider lowering it if this is a common issue for you.

Would you mind creating a new post with some details so we can take a look? I'd like to know more about your OS, browser, hardware specs, and what the size of the response payload is when you have the issue (the browser's network console will show you this).