If you’re getting output like this it means that page isn’t being loaded from the cache.
Page caching works by generating an HTML file from your output so future requests won’t touch rails and will be handled entirely at the web server level. By default, caching isn’t active in development mode so you won’t see any effect from it unless switching to production mode.
Once you have page caching working, improving performance works just the same as for any HTML page: reduce the number of images, javascript files, css files, etc, tune up your server, and things like that.
James.