I'm writing to draw your attention to a newly-available resource for performance analysis: flame graphs of MediaWiki code.
http://performance.wikimedia.org/xenon/svgs/
Flame graphs are a visualization of application stack traces.
Each application server on the Wikimedia cluster has a software timer that interrupts MediaWiki once every ten minutes to capture a stack trace. The stack trace shows what the code the application server was in the process of executing when the timer expired. A central log aggregator collects these traces and uses them to generate flame graphs.
Each box in a flame graph represents a function in the stack. The y-axis shows stack depth. The topmost box shows the function that was on-CPU at the moment the trace was captured. The function below a function is its parent.
The x-axis spans the sample population. The width of the box shows the total time it was on-CPU or part of an ancestry that was on-CPU, based on sample count.
Here is an example: http://performance.wikimedia.org/xenon/svgs/hourly/2014-12-15_22.svgz
To learn more about flame graphs and how to interpret them, see < http://www.brendangregg.com/flamegraphs.html%3E.