And I'm not just talking about operations that require network requests. I'm talking about simple UI responsiveness to (for instance) acknowledge a click and hold on a handle so I can start to drag-and-drop items in a list. I'm talking about drop-down menus that take over a full second to open.
I'm talking about lag when typing text into a UI, such that if you type a whole sentence and then click in the wrong place after you finish typing but before the second half of it renders out character by character (for instance, because you noticed and want to correct a typo), the second half of your sentence ends up transposed into the middle of the first half.
Ridiculous. This was a solved problem 25 years ago.