Slackware and Hyperbola GNU/Linux still run fast. Just pick XFCE instead of KDE under Slackware.

Deselect KDE if you don't need it. If the machine is old, it's better to use XFCE and install the rest later.

If you install and setup slapt-get you might install some nice KDE/Plasma software later to run under KDE. Then you can set the QT5 theme to GTK2 under /etc/profile.d/qt.sh (chmod +x it) and this content:

        export QT_QPA_PLATFORMTHEME=gtk2

Slackware is not 100% free but you can compile a libre kernel from FSFLA with ease and drop it into the UEFI partition or /boot and run the required grub/lilo/elilo command later.