The lag on the S6 is a major problem for me. I came from an iPhone 6, and while I prefer Android in many other ways, the s6 is much slower compared to iOS. It's really bumming me out. There's a very noticeable delay in almost everything you do on the S6 -- it's not a super long delay but is very noticeable if you are a super fast smartphone user constantly switching between apps. Every single S6 I have ever tried/used has this issue (I tried a bunch of them at two different Verizon stores, as well as four friends'), so I'm not encouraged by anyone saying otherwise here, I'm afraid...
Many people here recommended trying third party apps for the system UI, but none of the three biggest ones I tried made a single bit of difference with the lag. It's very amateurish-feeling compared to the iPhone and I'm regretting my decision to switch. It's too bad because I prefer Android in pretty much every other way, but I use my phone constantly all day for work and have become dependent on the truly immediate response of iOS. :-( Too much $$$ to switch to an iPhone (two-year contract dealie), so I'm hoping they fix this or I can bring it up to speed.
I commonly see Samsung being the brand most iPhone users head towards when they switch to Android because they all see Samsung touted as the main rival to the iPhone and in a sense they're right, it is - in sales. In performance, not so much other than the camera. If more of these people knew more about lighter skinned Android devices like Nexus, Moto, Sony I daresay you wouldn't see as many people who recently switched from their iPhone complain about software optimization. Yes, Samsung has scaled down Touchwiz, but it appears some problems still persist. One thing I can say is that the native keyboard app that comes with the 3 phones I just mentioned has performed quite well over the last few years.