If you notice your computer is slowing down and the fans are speeding up, after closing Lychee Slicer, open Task Manager. (CTRL-ALT-DEL) 

You might see that some Lychee Slicer processes are still running in the background despite having closed the program.

This is a known issue the team is working on fixing. Until this is resolved, the workaround is to manually end these tasks in Task Manager.