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
If you check cpupower configuration file it will probably show the default governor commented out with a # like this:
$ grep 'governor=' /etc/default/cpupower
#governor='powersave'
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
and change the setting for the governor to this:
# Define CPUs governor
# valid governors: ondemand, performance, powersave, conservative, userspace.
governor='performance'
For nano, use Ctrl + O to save the file and Ctrl + X to exit
Then use this command to start and enable the systemd service for cpupower to keep governor set to performance
systemctl enable --now cpupower.service
CPU frequency scaling - cpupower | ArchWiki
CPU governor randomly switching to Powersave, thus heavily limiting performance | forum.manjaro.org