S3 and its iteration of touchwiz were absolutely terrible..
The Galaxy S3 is actually the biggest selling Galaxy device of all time.
The S4, 5, 6, 7 have all sold far less ...
It shows how people's expectations from a device and moreover the plethora of choices we have now have affected phone sales compared to what were the 'heady' days back then. Even when software was iffy and improved greatly since.
Actually TW is only really beginning to break from its shackles since S6/7. The S4 TW was a mess of bloated features that caused the device to lag badly, I remember switching from the TW S4 to the Google Play Edition, the performance was night and day different. The S5 is best forgotten for more reasons than its software.
The S6 TW was first sign Samsung was starting to strip back TW but it was still bloated and had the ram issue (alleviated somewhat now with Marshmallow) and S7 has refined it more although still compared to other OS skins it does use up more ram resources by a significant margin. However at least with 4gb it curtails that heaviness somewhat.