4. Figure out browser caching - what to cache, how and for how long. Frequently changing code files - not a good idea to cache them for long, but images and other files, in most cases you only really need to download them once to the user's browser. Stuff to Google: etags and last-modified.
Improving site speed is a broad topic, and there would be other stuff on the server-side too where improvements can be made, like cached queries and using "prepared statements" to optimize the SQL.
Improving site speed is a broad topic, and there would be other stuff on the server-side too where improvements can be made, like cached queries and using "prepared statements" to optimize the SQL.