I don't care much about speed, but I absolutely care deeply about reliability. My 6 Plus is maddeningly unreliable. The tiniest bit of skin oil on the Touch ID sensor or finger and it doesn't work. The same happens to my wife's 6 and our son's 5s. And I'm a clean freak -- no dirty french-fry-and-salt-laden fingers here.
The question is will it work better with sweaty hands? Currently, I have a iPhone 6 and either I have a defective touch ID or it just doesn't work with my hands 70% of the time (more likely). Hopefully, there is going to be a better solution going forward as touch id is expected to be used more and more. When I see it working like this for "normal" people, I feel incredibly envious.
I found Touch ID so unreliable on the 6+ that I stopped using it after a couple weeks. And I keep my phone very clean. It would work great for about a day after setting up the fingerprints, but then would become unreliable until I deleted and re scanned my fingers.
When it did work, it was very fast, almost instant actually, but deteriorated quickly.
I'm more interested to know if they've made it more reliable, so I don't have to re-enter my prints every couple days.
I experienced all these issues on my first iPhone 6. Got it replaced, and never had a problem with Touch ID after that. Now on the 6s Plus, it's practically perfect.
So if you're not considering upgrading to a 6s/Plus, getting your phone replaced might do the trick.