I can confirm this behaviour, using Lubuntu in a VM when killing lxsession CPU usage drops down to usual levels and logs the user out of Lubuntu. Upon logging back in lxsession resumes using 100% CPU.
I have also marked bug 796843 lxtask shows 100% CPU usage regardless of actual usage as a duplicate.
I can confirm this behaviour, using Lubuntu in a VM when killing lxsession CPU usage drops down to usual levels and logs the user out of Lubuntu. Upon logging back in lxsession resumes using 100% CPU.
I have also marked bug 796843 lxtask shows 100% CPU usage regardless of actual usage as a duplicate.