That was a basic CPU Governor issue.

Most likely CPU Governor was misconfigured in Ubuntu, while in OnDemand in Mint.

That can easily be tweaked by this command:

sudo cpupower frequency-set -g GOVERNOR

Change GOVERNOR to one of the following, depending on what you want:

performance

powersave

userspace

ondemand

conservative

And it’s done.

You can check actual mode with:

cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

Hope it helps.