"lag preceptions" are just that, and are subject to personal opinion.
I have had mine since Launch day. During that time, there was "lag" it seemed when I wanted it to do something.
but, I have always been able to resolve that by finding the real culprit and getting rid of it.
For me, taking 3 seconds to do something might be a minor inconvienence. For someone else who is all "antsy pantsy" that might be a major issue because the desired action did not occur within 0.125 seconds.
The S5 and all new generation cellphones are just pocket sized computers, and the more tasks that we assign to them, the more time it takes to process everything.
ie, the more background tasks you have running, the slower your pocket computer is going to be in reacting to your request.
For myself, I minimize the background tasks severely. I selected the option in Settings to "Close each app after use". That has good and bad side effects.
The good side is, fewer background tasks means that when I ask the S5 to do something, it is fast.
The bad side is, each time I start/restart an app, it has to start up from scratch. ie, load into memory and then start processing the job. (for me, this is not worthy of discussing... I don't care)
Each cellphone when designed has to meet the engineer's priorties. Those priorites might not be your priorities.
I suggest that you spend some time at Best Buy or other store and compare how the S5 does the same jobs that the other "highly touted gizmo" does.
Bottom line for me is, the S5 does not have any significant amount of lag. Mine is optimized to my requirements and out performs my desktop computer for many tasks. The downside of the S5 is the screen is a lot smaller than my desktop's 24" monitor... but I just can't imagine trying to drag a 24" monitor around with me all day, can you?