www redirect

I want my website to appear as "www.domain.com" instead of "domain.com", whatever the user inputs to browser.
I tried this through .htaccess file with following code

Code:

---

Redirect / http://www.domain.com

---

Unfortunately this ceased to display my website.

Try this

Code:

---

RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]

---

http://www.neohide.com/htaccess-mod-...in-name-prefix

I have already used this script lines. And yes it's working perfectly but one thing I should mention is the site got slow down. Is there anything can be done for improving the site load time..