Mouse cursor lags behind after screen lock / user switching

I recently created a second user account on my computer due to somebody else needing to use it.

It appears that when I lock my session to let them open one with their account, the mouse cursor responds to user input with a significant delay : the cursor starts moving several seconds after the mouse, and also stops its course with delay. With some training, one might be able to use such a device but it really is impractical.

There has been several questions about this, but they are old, and the proposed solutions do not work in my case.

If I boot the computer, have them open a session, then lock it, then open my session, then they do not have a mouse cursor problem but I have. In fact, this happens in lightdm too.

However, if one switches back to the first opened session, that one still works without a problem. Which I find rather puzzling.

In any case : the graphics driver is NVIDIA 396.24.02, kernel is 4.15.0-24-generic and boot options are ro vesafb.invalid=1 nvidia-drm.modeset=1 nofb drm_kms_helper.fbdev_emulation=0 video=DP-1:3840x2160 rdblacklist=nouveau.