什么是关于在性能方面对脚本进行故障排除的好方法（php / mysql）？

I've written a site CMS from scratch, and now that the site is slowly starting to get traffic (30-40k/day) Im seeing the server load a lot higher than it should be. It hovers around 6-9 all the time, on a quad core machine with 8gb of ram. I've written scripts that performed beautifully on 400-500k/day sites, so I'd like to think Im not totally incompetent. I've written a site CMS from scratch, and now t