I had a particular branch of code where a typo caused a recursive stack error on 486 different tests. In the old UI, it took 19 seconds to load the page. In the new UI, it took 3 minutes (!) to load the same page. And any interaction with the page after it did load would be extremely slow. All that was after a couple tries where my browser just became unresponsive. This is because the new UI insists on showing all error details to begin with, rather than just showing the first line of the stack trace. This makes it totally unusable in a situation like I’ve described here, since the rendered page contains literally millions of lines of text.
I will add that I find it much less usable from a UX perspective to show all the details of every error by default, in addition to the much worse performance.