First check if you have mod_deflate installed by using phpinfo(), then try http://www.ipmango.com/http/deflate to check for relavant headers.
Besides deflate there are are couple of reasons for your site to be slow. All these tips are to be used with w3total cache.
- Try using varnish on your server, it has a huge effect on the page load as it already has all the content cached and serves them directly without going through the whole compilation. Its a bit pain if you have a site with constantly changing content.
- If varnish is a valid choice for your website, memcache is a must, you might already have it on your vps. If not try installing it.
- Css Minification is very important, try using the auto minification option, which will minify and combine them in a single file. And use of CDN will really speed things up.
- Use a speed optimizaton checking tool like http://tools.pingdom.com/fpt/ which will really let you know which things are causing issues on your site.