Your users are happiest when they don't notice how fast your application responds. If your users notice your web application speed it is probably because they notice it is too slow. Most speed improvements happen outside the Rails stack.

Typical page loading time

Here is the output of the Safari Web Inspector - Network report. The page took 3 seconds to load.

Notice that the Rails application took only 300ms, including the time needed to download the generated HTML. About 90% of the time was taken loading the various assets: javascript, css and images.

Tools of the Trade

YSlow

The primary tool for analyzing the speed of any web application is Yslow from Yahoo. YSlow grades your application and produces a report:

Safari Develop Menu

Install the Safari Develop Menu with the following command:

$ defaults write com.apple.Safari IncludeDebugMenu 1

The Safari Web Inspector - Network report will produce a report like above showing the amount of time spent loading various components of the page.

Make fewer HTTP requests/Minify JS

You should send only 1 javascript file and 1 css file. No more. The asset packager plugin is a plugin that will automatically package all of your css files and javascript files into one. Install with:

$ script/plugin install git://github.com/sbecker/asset_packager.git

Then produce the asset_packages.yml with:

$ rake asset:packager:create_yml

You will need to edit the config/asset_packages.yml file to ensure that the scripts are loaded in the correct order.

In your application.html.erb layout file place the following:

<script src="http://ajax.googleapis.com/ajax/libs/prototype/1.6.0.3/prototype.js" type="text/javascript" charset="utf-8"></script>
<%= javascript_include_merged :base %>
<%= stylesheet_link_merged :base %>

The prototype.js file will be loaded from Google. With any luck the user will already have it cached.

Load files in parallel

Most browsers will make at most two simultaneous requests to each host. Configure your production.rb to use an assets host:

# Enable serving of images, stylesheets, and javascripts from an asset server
config.action_controller.asset_host = "http://assets.yourhost.com"

Of course, you will have to configure assets.yourhost.com to point to your web server.

Add an expires header/Gzip components

If you are using Apache you can configure your virtual host directive to use mod_deflate and also mod_expires:

<VirtualHost *:80>
    ExpiresActive On
    ExpiresDefault "access plus 1 month"
    AddOutputFilterByType DEFLATE text/html text/plain text/xml application/javascript text/css
    ...
</VirtualHost> 

Etags

Rails 2.2 supports Etags. Look for another post about Etags in the future.

More reading (update)

For a lot more detailed discussion about improving your sites performance see: Stanford Course CS193H: High Performance Web Sites.