1. disable Transparent Hugepage Support, by adding the kernel argument transparent_hugepage=never. on Debian-adjacent distros, this can be done by putting this in /etc/default/grub.d/no-hugepage.cfg:
GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT transparent_hugepage=never"

then run update-grub

2. run these:
git clone https://github.com/jemalloc/jemalloc
cd jemalloc
./autogen.sh
make
sudo make install

3. either put this on top of ~/.xinitrc:
export LD_PRELOAD="/usr/local/lib/libjemalloc.so $LD_PRELOAD"

or, if you want to be more thorough (be careful, this is harder to recover from, should it go wrong), put this in /etc/environment:
LD_PRELOAD=/usr/local/lib/libjemalloc.so

4. depending on how you did step 3, either restart xorg or reboot your system.