well, I think that if .html is enabled as a PHP extension, non-php html pages may load slightly slower, though when I tested with my own webserver this didnt seem very noticeable, if it is true.
in my case I tested with apache, so I changed httpd.conf from
AddType application/x-httpd-php .php .phtml
to
AddType application/x-httpd-php .php .phtml .html
I'm not much of a system admin, so I cant describe in detail what one should do.