UI lag is a problem, for sure -- and so are some of the dynamic layouts that are meant to "solve" it[1].
One thing I've noticed lately is mouse lag. Like, say, in Ye Olde Start Menu on Windows: Move the mouse to the Start menu, and press the mouse button down. Nothing happens. Release the button, and then: Something happens.
The menu is triggered on button-up events, not button-down events. This adds a measurable delay to every interaction.
Same with Chrome when clicking on a link: Nothing happens until the button is released. This adds a delay.
I mean: Go ahead and try it right now. I'll wait.
And sure, it might be a small delay: After all, it can't be more than a few milliseconds per click, right[2]? But even though the delay is small, it is something that is immediately obvious when opening the "Applications" menu in XFCE 4, wherein: The menu appears seemingly-instantly when the mouse button is first pushed down.
[1]: It took me more than three tries to pick a streaming device from Plex on my phone yesterday. I'd press the "cast" button, and a dynamic list of candidate devices would show up. I'd try to select the appropriate candidate and before my thumb could move the fraction of an inch to push the button, the list had changed as more candidate devices were discovered -- so the wrong device was selected.
So I then had to dismantle the stream that I'd just started on the wrong device, and try (and fail) again.
[2]: But even small delays add up. Let's say this seemingly-unoptimized mouse operation costs an average of 3ms per click. And that 100 million people experience this delay, 100 times each, per day.
That's nearly an entire year (347 days) of lost human time for the group per day, or 347 years lost per year.
Which is 4.4 human lifetimes, per year that are lost within the group of 100 million, just because we're doing mouse events lazily.