Is it only at launch? Afterwards you have no issues? I don't own an S6 but a friend does and he doesn't seem to have that problem. HOWEVER, a lot of T-Mobile Note 4's (mine included) had an autofocus issue that was hardware related. I exchanged mine after a month and the replacement worked just fine. Maybe the unit you have has a similar problem?
I've noticed it takes a bit of time focus in low light (or sometimes not even until you tap it) but in normal lighting it focuses right away.