I think that Quantum splits up the task of rendering web pages between multiple processes on multiple cores of your processor. It means that Firefox is now the fastest browser (or at least faster than IE, Edge, Chrome and Safari) but it uses one heck of a lot of processing power. I guess that sharing the task of rendering inevitably results in some inefficiency, in addition to the overall increase in speed, too. I was getting blue screens of death when other things used up a processor's capability to respond to sync calls. Turning off Firfefox's hardware acceleration in settings makes Quantum as slow and peaceful as Firefox 56. That is what I have done. I don't need the speed. That said on other machines I don't need to turn off acceleration and FF runs at a blistering speed, so I guess that there is some BIOS / chipset driver or memory problem on the machine that gets BSOD.