I just upgraded to -current, which should become Slackware 15.0 at some point. I immediately noticed that hinting was awful. I tried changing the settings in Xfce, but there was no improvement. Finally, I realised that it should be set up elsewhere.

I needed to set it up in /etc/profile.d/freetype.sh. I opened the file and uncommented the following line:

#!/bin/sh
# Configure Freetype properties. Here this is used to set the default mode
# for font hinting. Other controllable properties are listed in the section
# 'Controlling FreeType Modules' in the reference's table of contents.
#
# Three hinting settings are available:

# This is the classic hinting mode used in Freetype 2.6.x:
export FREETYPE_PROPERTIES="truetype:interpreter-version=35"

# This is Infinality mode, which was never enabled by default. It is slower
# than the new subpixel hinting mode, but said to be more accurate:
#export FREETYPE_PROPERTIES="truetype:interpreter-version=38"

# This is the new default subpixel hinting mode used in Freetype 2.7.x. It is
# derived from the Infinality code base stripped to the bare minimum with all
# configurability removed in the name of speed and simplicity:
#export FREETYPE_PROPERTIES="truetype:interpreter-version=40"

Also, many of the traditional fonts, such as Monospace, somehow look like crap in the text editors. I ended up specifying the DejaVu fonts, instead.