S Voice is one of those features that Samsung has had on their devices since the Galaxy S3. Which was their first smartphone they launched after the iPhone 4S which brought Siri into the game. S Voice was never all that great, but there are people that prefer it over Google Now. If you double tap the home button, it'll launch S Voice, and long-press will do Google Now. So why am I talking about the home button? Well the Home button lags a bit because of S Voice. The software is waiting to see if you're just tapping it once or tapping it twice for S Voice. So you can speed that up a bit if you're not using S Voice, I'd suggest disabling the shortcut. Which is exactly what we are doing today.
Simply go into Settings and tap on the search icon. Now type in "S Voice" and choose the first result. This will take you to the S Voice settings. From there, go down to the section called "Wake-Up". From there you can uncheck the "Open via the Home Key". Which will disable the double-tap of the home button to launch S Voice. Now if you like the double-tap then I'd suggest not getting rid of it. But if you want your Galaxy S5 to be a bit faster, that's the best way to do it.