If your processor is defaulting to using the powersave CPU frequency scaling governor, then it is probably using the intel_pstate CPU frequency scaling driver. Check via:

$ grep . /sys/devices/system/cpu/cpufreq/policy*/scaling_driver

/sys/devices/system/cpu/cpufreq/policy0/scaling_driver:intel_pstate

...

/sys/devices/system/cpu/cpufreq/policy5/scaling_driver:intel_pstate

Ubuntu has changed the default kernel configuration as for the default governor. It used to be performance and now it is schedutil, which will fall through to powersave if schedutil is not available. Therefore, some of the older answers no longer apply.

Method 1
The ondemand service just calls /lib/systemd/set-cpufreq, which could be edited to set the governor to performance instead of what it currently does. This old answer indicates a method, repeated and modified herein:

doug@s18:~/config/lib/systemd$ diff -u set-cpufreq.original set-cpufreq

--- set-cpufreq.original        2021-03-10 14:07:32.036863542 -0800

+++ set-cpufreq 2021-03-10 14:10:05.313627963 -0800

@@ -10,6 +10,10 @@



 read governors < $AVAILABLE

 case $governors in

+        *performance*)

+                GOVERNOR="performance"

+                break

+                ;;

         *interactive*)

                 GOVERNOR="interactive"

                 break

After the edit and after re-booting, check it:

$ grep . /sys/devices/system/cpu/cpufreq/policy*/scaling_governor

/sys/devices/system/cpu/cpufreq/policy0/scaling_governor:performance

...

/sys/devices/system/cpu/cpufreq/policy5/scaling_governor:performance

And check the status of the service, which should be dead by now:

$ sudo systemctl status ondemand

[sudo] password for doug:

● ondemand.service - Set the CPU Frequency Scaling governor

     Loaded: loaded (/lib/systemd/system/ondemand.service; enabled; vendor preset: enabled)

     Active: inactive (dead) since Wed 2021-03-10 14:13:02 PST; 1min 18s ago

    Process: 667 ExecStart=/lib/systemd/set-cpufreq (code=exited, status=0/SUCCESS)

   Main PID: 667 (code=exited, status=0/SUCCESS)



Mar 10 14:12:57 s18 systemd[1]: Started Set the CPU Frequency Scaling governor.

Mar 10 14:13:02 s18 set-cpufreq[667]: Setting performance scheduler for all CPUs

Mar 10 14:13:02 s18 systemd[1]: ondemand.service: Succeeded.

If something later on during boot is overriding the governor setting it would be by far best to figure out what and get rid of it. However, and just as a temporary workaround try introducing a sleep delay into this service (note the older ondemand startup script used to delay 1 minute, then change the governor). Untested example:

doug@s18:~/config/lib/systemd$ diff -u set-cpufreq.original set-cpufreq.doug.test

--- set-cpufreq.original        2021-03-10 14:07:32.036863542 -0800

+++ set-cpufreq.doug.test       2021-03-10 16:24:13.088946203 -0800

@@ -10,6 +10,10 @@



 read governors < $AVAILABLE

 case $governors in

+        *performance*)

+                GOVERNOR="performance"

+                break

+                ;;

         *interactive*)

                 GOVERNOR="interactive"

                 break

@@ -34,6 +38,8 @@



 [ -n "${GOVERNOR:-}" ] || exit 0



+sleep 60

+

 echo "Setting $GOVERNOR scheduler for all CPUs"



 for CPUFREQ in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

Method 2
Now, if you prefer to run with that service disabled:

$ sudo systemctl disable ondemand

Removed /etc/systemd/system/multi-user.target.wants/ondemand.service.

then the preference can be set on the grub command line, by adding cpufreq.default_governor=performance to what is already present. Keep a copy of /etc/default/grub before you start and in case you want to revert later. This example includes other stuff already in my command line. So, in my case, I changed this:

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 consoleblank=450 msr.allow_writes=on cpuidle.governor=teo intel_idle.states_off=4"

to this:

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 consoleblank=450 cpufreq.default_governor=performance msr.allow_writes=on cpuidle.governor=teo intel_idle.states_off=4"

Run sudo update-grub afterwards and re-boot. Then check:

doug@s18:~$ grep . /sys/devices/system/cpu/cpufreq/policy*/scaling_governor

/sys/devices/system/cpu/cpufreq/policy0/scaling_governor:performance

...

/sys/devices/system/cpu/cpufreq/policy5/scaling_governor:performance

Caution: Be sure your computer can run in performance mode without creating to much heat, because whatever thermal throttling methods are used might not be operating yet during the boot process.