Antony Dovgal has announced a tool that can help you find out where issues are in your code - specifically places where too much work is being done and gumming up the works. The Pinba statistics server for PHP that gathers UDP data from the PHP processes and makes it available for parsing/graphing.

What is it? It's a daemon gathering information sent by PHP processes by UDP. In the same time Pinba acts as a read-only storage engine for MySQL, so you can use good ol' SQL to access the data. [...] There is no need to store that information for further analysis, therefore Pinba doesn't actually store the data - it keeps it only for 15 minutes (you can change that, of course), which is more than enough to update graphs.

You can find out more about the project on its (wiki) site including links to the latest downloads (version 0.0.3 at the time of this post).