Most of the time, slowness in Wordpress is related to shared hosting - most shared hosting uses a remote (also shared) mysql server - or theme-related, many includes, graphics heavy. However, that set of rules does search the entire file system twice - once to see if the request is a file, then again to see if it's a directory.
If you're using extensionless URLs,
This one works pretty well. The reasoning is that only your images, etc. will have extensions. So if it doesn't have an extension defined with a dot, to Wordpress it goes.
RewriteRule ^([^.]+)$ /index.php [L]