Warning for the general reader: this is a semi-technical article for WPMU admins and WordPress users hosting their own sites on services such as Bluehost.

We’ve just run into this problem, and we’ve solved it. Completely. The site which was running slowly is now bouncing along at an unflappable rate, and we still have a strategy or two to implement. Such a relief.

So here’s a bullet-proof list to go from choked to blazingly quick:

1. Don’t panic. There will be a really simple reason for it. And unless you have big traffic, it is unlikely to be WordPress or WPMU
2. Check the tmp file of your server — looking for mysql_slow_queries particularly. This will give you an indication of where things are going awry.
3. Look at your plugins, and turn all non-essentials off.
4. Look at your widgets Get rid of anything that looks like it might be cpu intensive. Like what? Tag clouds, related post lists, most commented lists. Anything that is dealing with big numbers of entries in the db on a constant basis.
5. Look at your overheads. We installed WP Optimize for our emergency, and it helped, cleaning out all those overheads. We’ll continue running it.
6. Look at your tables. If you have any tables in your db with big numbers of rows, here’s the likely cause of your problem. We had over 30,000 tags, 26,ooo or which had been used once or twice. Bin em, you won’t notice they have gone and suddenly your concorde rather than a puttering old cessna.
7. Look at your visitors. How many log in? How many just read and bounce out? Mostly the latter — so install Supercache and let the bouncers see cached pages rather than use precious db and php resources to serve them something that could be achieved in two or three calls to the server.
8. Optimize your theme.
   • replace anything starting with bloginfo() with the real deal eg. bloginfo(‘name’) becomes (in our case) ‘ForArgyll’. 
   • consolidate your css into one file
   • push your javascript into the footer, not the header
   • optimize and minimize all your images
   • forget having abstracted themes if you have huge amounts of traffic. It’s why the default theme is genius.
   • Don’t use widgets, or if you do test them.
9. Turn off Revisions. Do it. There’s a line in your wp-config file. The more posts WP has to cycle through the slower will the CPU go.
10. Test your plugins as you turn them on. They’ll tell you if things are awry. Just keep an eye on the cpu throttling app in the cpanel.

That’s all you need to know. Guaranteed. And if you’re still having problems, it may just be that someone is spamming you or your db.