Ok, gpu scaling, what it does it scales the resolution to pair the monitor. Since your monitors has different native resolutions you should check the option and should be ok.
It is known to sometimes have problems when maximizing and minimizing in games, if everything is ok you are set.
No, it should not be the only way to fix the blurryness, I believe modern monitors can auto scale on their own through options.
Make sure you have the native resolutions of both monitors set at that resolutions through ATI CCC or windows display manager.
It does not slow your computer at all.