You're describing exactly what makes them feel slow. All of that buffering is adding latency. Everything is buffering: the screen, the GPU, the OS, the window manager, the shell.

People notice the extra latency though. I sure do. I remember what it's like to have a CRT getting photons displayed nearly immediately after a keystroke. That's exactly what makes an old 286 feel snappier than a 2017 macbook pro while typing.