I think I'm seeing same/similar issue. I generate CLOBs with XML data. Opened a result in Large Data Editor window. There is a little bit lag in opening the window, but then I can see the data, the size (about 4,5 MB in this case). One can see it is all loaded in the window. Can scroll down to the end of the text.

I select all and hit Ctrl-C. App freezes. It has now been running one core at 100% for 20+ minutes. I recently updated from 14, and while I noticed a significant delay there also, it was nowhere as close.