Are you sure it is a .htaccess rewrite issue? Is your main site also sluggish? Your .htaccess file contains a *lot* less code than mine. And my site isn’t exactly slow.
I timed http://ochremusic.com/ using my wristwatch. The main page takes 14 seconds to load. 6 seconds for the site to even begin loading. Clicking forums immediately took me 5 seconds for the page to load.
That makes it 6 seconds for the DNS to resolve(?) 8 seconds for the main site and 5 seconds for the forums.