How to regain speed on older Intel machines (insecure)

This disables in-kernel Spectre and Meltdown mitigations that have a significant impact on performance but leaves your machine disarmed against such attacks.

sudo sed -i "s/GRUB_CMDLINE_LINUX=\"\"/GRUB_CMDLINE_LINUX=\"mitigations=off\"/" /etc/default/grub
sudo update-grub
reboot