Cinnamon desktop lagging, spiking CPU every other second

I asked this question first on the Linux Mint Forum, where mr_raider provided the solution. All credits go to him.

The problem was caused by the default AMD drivers. The solution is installing an updated version, the oibaf AMD drivers. The procedure:

sudo add-apt-repository ppa:oibaf/graphics-drivers
sudo apt update
sudo apt upgrade

After this and a reboot, the lag is no more.

The forum thread is available there:
[SOLVED] Cinnamon lagging, causing CPU spikes every other second