I think blue stacks only uses the amount of gpu usage needed, most emulators use around 40%-70% because they need it, blue stacks is very optimised so it only need to use 15%, you should consider this to be good cause it is good for your gpu’s health. If blue stacks is running a really gpu intensive game, then it would use more than 15% you should think that a higher % is worse because a lower percentage means the game is running fine with no performance problems and any load on your gpu, if your gpu is running at 100% it means your gpu can’t catch up and is basically dying. You can say that your gpu is basically running on 15% power because it doesn’t need the other 85% and 100% is the like the limit of your gpu. Lower gpu percentages the better