However, I have never analyzed which are the exact causes, because my configuration has a lot of differences and I am not sure which of them matter.

I suppose that it is important that I use neither Gnome nor KDE, but XFCE. Even so, some of the applications that I use are intended for KDE or Gnome, so they use the corresponding libraries, and I do not know if these have any reason to be more responsive under XFCE than under their native desktop environments.

I prefer to use a completely empty desktop, without icons or toolbars and having as background a neutral gray. I launch applications by right click on the desktop and I restore minimized windows from an auto-hidden taskbar.

I take care that there are no active services/daemons besides those that I really need. I do not use systemd or anything associated with it. I have stopped using swap memory two decades ago (but I equip all my computers with generous amounts of DRAM, to avoid out-of-memory situations; my oldest computers have at least 32 GB and nowadays I would not buy one with less than 64 GB).

I use only 4k monitors with 30-bit color, so using lower resolutions is not needed for a snappy GUI.

I use the proprietary NVIDIA drivers or the Intel drivers on computers with the integrated Intel GPU. I use a customized Linux kernel, but I doubt that this can have any influence on GUI responsiveness, even if it ensures fast boot times. I do not use Wayland and I doubt whether it is the right replacement for X Window, because its initial design was very flawed, even if some of the original defects have been corrected meanwhile.

For viewing PDFs or EPUBs, I use MuPDF, which is much faster (especially on startup, which is instantaneous) than the other viewers that I have tried. It has some limitations, so I also keep around other viewers, e.g. Okular, for the cases when I need a feature not provided by MuPDF. As file manager, I use Xfe.