My internet mysteriously slowed down specifically on EndeavourOS last week, it has not sped up again since

Thanks @dbarronoss

I am not running Plasma so I can mistaken with the login manager.

I see sddm in the inxi that’s why I suggest enable sddm service.

System:
  Kernel: 6.19.10-arch1-1-bsb arch: x86_64 bits: 64 compiler: gcc v: 15.2.1
  Desktop: KDE Plasma v: 6.6.4 tk: Qt v: N/A wm: kwin_wayland dm: 1: LightDM
    note: stopped 2: SDDM Distro: EndeavourOS base: Arch Linux

The main thing is trying disable/remove lightdm as login manager and use the plasma login way.