Depends on who you ask...
Some folks have experienced this, others not so much. But it could be highly dependent on usage pattern between users. Certainly there are more reports of this happening than on stock android or Sense.
Likewise you always have that risk that they botch something in a Touchwiz update that makes your device perform like its wading in treacle (such as my lollipop note 4 update). But again this can affect some users and not others.
So yes there is always the potential of touchwiz being a complete pig, and probably more chance than on other android skins - but it's still not a vastly common issue as it once was a few years back. Indeed 2014 Touchwiz onwards generally has been the smoothest it's ever ran, with caveats of course.
Basically if they have improved not so much on the smoothness of the GUI (for most part since S5 the GUI on top tier Samsung devices has been very smooth) but the memory management side (it's here under the hood where touchwiz can be most culpable for the often reported lag and refreshing issues) then I wouldn't be too concerned. The S6 has more ram then the S5 so the app refreshing it suffered shouldn't be an issue on the S6 (in theory). So yeah as long as you don't get a dodgy firmware update that banjaxes that like the DBT Lollipop update I wouldn't be too concerned about it slowing down over time.
But.... Before taking that as gospel, I do however want to see exactly what memory the S6 uses at stand still.... I am concerned that of all the early hands on reports and even GSM Arena review not once has someone reported what memory is used by the system and how much is left to the user. That concerns me that there is so much silence in this regard, from even the likes of Erica Griffin who normally goes very in depth and GSM not even covering it - in case as often there is - a NDA clause about that reporting on that side until it's 'final retail software'....
So yeah wait and see and if your unsure - just wait till someone here picks one up and user reports begin hitting the net. If there are issues - users will report them likely sooner than the tech journalists / bloggers these days...
Thanks. Very informative.
As much as I want the S6, it probably isn't a day one buy for me. I'll await reviews and feedback. I'm quite happy with my OnePlus, but do want some of the advantages the S6 offers. Namely, better camera, wireless charging, and even little things like the headphone jack being on the bottom again (like it was on my iPhone 6).