Here's some quick benchmarks I threw out.
With NO PHP code in the file, and changing the extension from php3 to html, this is what I get:
byte/s rate for 1000 copies via "ab -n 1000 server/page.{html|php3}"
Size(bytes): 1k | 20k | 60k
html 880k 5.5m 15.5m
php3 309k 1.4m 2.8m
As you can see, while there is a penalty for adding PHP, it's not a real show stopper.
If you really want to add PHP interpretation to an apache server, find the line that looks like this in httpd.conf:
AddType application/x-httpd-php .php3 .php
and add .html to the end of that line and restart apache (apachectl restart)
The better answer is to find the entry in httpd.conf that looks like this:
<IfModule mod_dir.c>
DirectoryIndex index.html
</IfModule>
and make it look like this:
<IfModule mod_dir.c>
DirectoryIndex index.html index.php3
</IfModule>
Then just make your index page be index.php3 and you're done!