Actually Bluestacks code is pretty unoptimized.
And Bluestacks will start with lots of ads and auto-download random apps constantly. A large % of extra processing power is eaten up by Bluestacks memory leaks, random ads, and continual use of your bandwidth for non-app related purposes.
In fact, typically I start an app after it is just about fully loaded, Bluestacks kills the app to load another full page ad and I have to repeat the loading 3-4x before the app actually runs. Or unfortunately more often than not, instead of letting the app run, after killing the app, Bluestacks will sit at the loading screen forever. Kind of annoying but it how it works.
Also when you are done with Bluestacks, reboot or use Task Manager to kill all the Bluestacks background processes (between 4 to 16 depending on which version and how much you can see with Task Manager).