There's some good advice in there, and I thought it'd be a good idea to quickly add a bit more detail about the separate approaches that Mike raises.

He goes over the APC caching, memcache, the "gzip trick", the "Not Modified" header and optimized SQL statements.

He also mentions one thing that Mike didn't mention - a split between static files (no PHP needed) and their dynamic cousins. Having a more pure Apache (no PHP installed) can help give a minute jump in speed that, depending on the size of the site, could really add up from a user's perspective.

There are 4 relatively easy ways by which you can speed up the time it takes a browser to download a page. Following on from my post on joining CSS and JavaScript files, I have written a PHP script which will automatically do all of the above.

He gives example code of the class in action and includes some screenshots of the benchmarks from the YSlow Firefox extension showing the improvements.