You will find a lot of posts stating that the S5 has horrible lag.
Those posts are written by folks who think every thing they click on should happen "instantly". They don't have a clue as to what is going on behind the screen of the phone in their hand.
Adding a lot of "gee whiz" features has problems that will show up in slowness.
ie, the cutesy wallpapers take time to draw.
The transition effects from one screen to another screen takes a finite amount of time. If you want cutesy effects when that occurs, then you are going to experience a time lag. Turning OFF all of the background animations eliminates lag between screens. That is my choice, might not be your choice.
The "fastest wallpaper" is pure black. Why? because nothing has to be drawn on the screen. No mathematical algorithms going on to create those cutesy whirly birds, etc.... Just an instantaneous 'click' and it is black.
Background applications that are constantly polling their hosts to get updates will slow down the internet access which results in overall phone slowness. I choose to not have any background apps running period, my phone is fast because none of my apps can access the internet until I tell them to do so. I purposely turn OFF data/wifi when I'm not using my phone. You most likely won't, so YMMV with how fast your phone is compared to mine. And we will be using the same model S5.
Configuration is extremely important, and that means how you set up the OS to respond, and how many apps you install, and how those apps interact with the phone/internet.
So, read as much as you can on the forums, but in the end, any phone you choose can work well.
Just be sure you won't be experiencing "Buyer's Remorse".
I personally can only afford one phone every 2 or 3 years. Getting the next one is crucial for me, I only get one chance to "get it right".
so far, the S5 has been a "good phone". It could have been better though. I wish the Camera were "instantaneous" and ready for the next shot within 1/4 second of taking a shot. It isn't and it never will be, but it is "good enough". I choose to not get into a dither about it because we can't change it. We can choose to turn off features that add slowness, but that is the best we can do.