Yeah the GS 3 is definitely superb for comfort in the hand especially considering the size. No reason why they can't keep the ergonomic curves but use a better finished plastic and improve the overall design though!
My one S is extremely fast and smooth, no lag or slowdown at all, only place I get some lag, is with the google play store when scrolling through the app list. The beauty of android as well, is that you change the animation speed or animation so things are either snappier or smoother. The only android phone, which has truly beat the one S (from my experience) when it comes to overall performance is the nexus 4. iphones are very smooth, but I don't find them that much, if any better at all, certainly not enough to worry about imo anyway.
Plus considering all the different android phones out there with different hardware (not to mention being more demanding than iOS in the sense there is more going on, multi-tasking etc.), I would say that google and the OEM manufacturers have done a pretty damn good job.
Before the GS 2, android phones were nowhere as smooth throughout but since the GS 2 and now jellybean, it is no longer an issue.
2GB RAM does make a difference especially if you like to browse with lots of tabs opened, although it isn't really "required/needed" for a fast/smooth phone, more of a "luxury" really.