I am considering Smarty as my web app templating solution, and I am now concerned with its performance against plain PHP.
The Smarty site says it should be the same
Smarty generates PHP code for all its template files when they're first used, provided you have it set up correctly, and uses them when possible instead of parsing the templates again.
I used it for a while and it was fast enough, but in the end I swapped it out for plain PHP files because it was a bit limiting (too many PHP4-isms).