I dunno.... I've see a few side by side comparisons with the Nexus 5.... and while the slightlyless powerful Nexus is a bit smoother and peppier than the S5, seems to me that the average user may not notice it all that much unless you held the two side by side.
Though, where the S5 DOES seem to have a noticeable disadvantage is multitasking. It was noticeably slower to switch between apps, and the heavier RAM footprint of TW was evident (there were fewer apps kept in memory to quickly switch to). But the fact still remains that the S5 is still a VERY fast phone.... sure, it might not be as responsive as a Nexus 5, or the M8, but it's not that far behind on most things and other than task switching, the difference isn't all that noticeable.