Package cpupower includes a systemd service to set the CPU governor at system boot, but the service is not active on Manjaro and has to be configured for the correct governor
Open the configuration file in a text editor
(other text editors are available, but nano is usually installed on any version of Manjaro)
sudo nano /etc/default/cpupower
add the setting for the performance governor
# Define CPUs governor
# valid governors: ondemand, performance, powersave, conservative, userspace.
#governor='ondemand'
governor='performance'
Use Ctrl + O to save the file and Ctrl + X to exit nano
Then enable and start the systemd service
systemctl enable --now cpupower.service