First thing - only use custom kernels when using Manjaro unstable branch - as this ensures the package level is matching Arch proper.
sudo pacman-mirrors -aS unstable && sudo pacman -Syyu
- [Need-To-Know] About Manjaro and AUR
- [root tip] [HowTo] Check if your AUR build scripts have been updated
If you get stuck on plymouth - first step would be to remove plymouth - and the splash configuration in mkinitcpio.conf and /etc/default/grub.
In the default grub - you can add kernel arguments
GRUB_CMDLINE_LINUX_DEFAULT="nowatchdog mitigations=off ... "
Then rebuild init and grub config.
If you are using nvidia gpu - use nvidia-dkms - instead of the precompiled nvidia in Manjaro repo or switch the gpu to AMD.
If you are seeking a kernel optimized for your system - change CFLAGS flags in ~/.makepkg.conf.
If you don’t have that file copy from etc
cp /etc/makepkg.conf ~/.makepkg.conf
First step would be to target your cpu specifically
-march=
[...]
CFLAGS="-marc=native ... "
[...]
in my case it is a ryzen so I use
-mtune=
[...]
CFLAGS=" ... mtune=znver3 ...."
[...]
Then there is the documentaion on Improving performance - ArchWiki