If you have a dedicated server or a vps with ssh access run this test from you command line.
ab -n 1000 -c 2
http://www.yousite.com/
Look for ?Request per second?
Run it a couple of times to get a benchmark of where you are at.
If you have shared hosting go here.
http://tools.pingdom.com/fpt/
This site will breakdown where the bottle necks are. Once you find out that info you can then systematically make improvements in that area. Use a search engine to find that info. I had to add a php-accelerator., mysql query cache, server cache and some cache control to my .htaccess file
Add this to you .htaccess file
# 480 weeks
<filesMatch "\.(ico|flv|pdf|jpg|jpeg|png|gif|js|css|swf)$" >
Header set Cache-Control "max-age=290304000, public"
</filesMatch>
# 2 DAYS
<filesMatch "\.(xml|txt)$">
Header set Cache-Control "max-age=172800, public, must-revalidate"
</filesMatch>
# 2 HOURS
<filesMatch "\.(php|html)$">
Header set Cache-Control "max-age=7200, must-revalidate"
AddDefaultCharset UTF-8
</filesMatch>
Restart your server or if you on shared hosting ask them to restart you and re-do your test.
Also if you have access to your php.ini check to see if you have the mod_deflate modul.
Good Luck