[xdude]

I usually use google pagespeed tool to check how fast my website load . And if the result is slow i will changing my theme to improve the result

If you are talking about Wordpress, then theme is not the only thing which could slow down your web site. There are many other things like,

• Plugins.
  
• Widgets.
  
• Scripts you running in Head or footer.
  
• Big images you use for header or background.
  
• CSS rules.
  
• Caching process.
  

etc etc.
Google pagespeed is a good tool but when it comes to Wordpress you can get better more detailed result from gtmetrix.com. of course you can run both and see the common problems shown by both tools. First thing to do is test your WP site with one of those default themes without installing any plugins and widgets. Keep a screen shot of this data and you can compare it with other things once you installed Plugins, themes, widgets etc.