I also noticed lag when playing around with the GS5. its a samsung thing. The LG G2 never lags.
Quotes like this led me to getting an LG G2. However, in my opinion it does lag. Try the gallery app with a decent amount of pictures/videos in it. Try opening the phone app and swipe across for recent contacts and watch the lag/delay. Try the music app and swipe across and watch the lag/delay.
This isn't major lag but for top end phones I find it odd how the basic functions are stuttering/lagging on such powerful hardware. Also if you haven't spent time with a system that doesn't lag or stutter like that you won't have a proper reference point.
My iPhone 5 does these tasks effortlessly.