OK I have an old PC and Unity Light baking is slow for me but not only is it slow it’s a CPU hog.
So is there a way to reduce the CPU load while baking so I can go off and do something more interesting on my PC without it grinding to a halt or crashing?
Change task priorities in the windows task manager.
Lightmapping sometimes make the system slower even on high end CPUs. If you had more powerful CPU, Unity use more power of CPU and bake faster. Your system still slow.
I have tested it on different systems .
Will this work across sessions or need applying every session?
It needs re applying every bake I’m afraid. 
I think this will mostly change in future with the iterative baking Unity’s got planned so it’s a case of just waiting.
In any case I think it probably would be a good idea to have a % throttle on it of some sort in prefs.
scaledown your map, 0.01 usually work best for me, including all lights range before baking, and scale them back after baking is done…