I found that using Smarty with PHP, sometimes extra time will need to be used for
1) using quite different syntax than PHP itself
2) need to check small cases, b
As far as I know Smarty is one of the best template engines speed wise. Maybe it takes a while to get used to it. But if you are not working on the system alone and the amount of html and style files is huge it speeds up the development significantly.
While working on my last project the design was changes a couple of times but logics was the same. I guess that's the best example when Smarty or any other template engine helps a lot.