Between all th caching of the OS, web server, and php, templates can run nearly as fast as non-templated pages, and are MUCH easier to maintain.
Much more important is writing efficient php code outside of your templates. If you have even one sql query in a page, that you parse into a table, that alone will likely cost you more in CPU than all the other stuff required to make the page from templates versus non-templates.
Compared to math, long loops, sql queries, and other complex stuff, tempates are a cheap way (in CPU power) to make better use of your time and build a more resiliant web site.
Of course, it all comes down to how much your time is worth versus the amount of CPU horsepower it takes to use templates.
