I often need to browse around / edit / reduce minified HTML, JS, and CSS files from websites.

Such files have the property that they can have lines that are tens of thousands of characters long.

Kate's performance when working with such lines is terrible. For example, when using Shift+DownArrow to select several such lines, the response time can be in the tens of seconds per keystroke. Copying/pasting many such lines has sometimes caused Kate to hang for minutes, and I've been forced to kill it. Clearly, Kate is doing something that's quadratic or worse in the line length or selection length.

There are other text editors, like JEdit, which handle such long lines perfectly fine, so it's definitely possible.