I used unclutter on Raspbian Buster to hide the mouse pointer. I did not directly notice, but indeed after using unclutter lxpanel also had ~100 % CPU usage. Somehow they disturb each other. 

I simply uninstalled unclutter and hid the mouse pointer by another method:

        sudo nano /etc/lightdm/lightdm.conf

        changed:
        # xserver-command = X

        to:
        xserver-command = X -nocursor

and lxpanel had a nice low CPU usage again.