I got curious and installed Lubuntu as well. By default, I'm experiencing screen tearing in the OS. It seems like there's no hardware acceleration in browsers because 4K videos are choppy. However, when I enter about:support
in the Firefox search bar, I see "Compositing: WebRender," which indicates the presence of hardware acceleration
You can check hardware acceleration state at about:support page, look at Compositing row. If there's WebRender, you're running on hardware. If there's WebRender (software) you're on non-accelerated backend.
(https://fedoraproject.org/wiki/Firefox_Hardware_acceleration).
But it seems like something is not quite right.