Derick Rethans has a new post to his blog today talking about some work that's been done to speed up XDebug's code coverage generation. Changes in the coming 2.2 release have some improvements that make things perform better and put less stress on PHP in the process.
Code coverage tells you how much of your code base is actually being tested by your unit tests. It's a very useful feature, but sadly, it slows down PHP's execution quite a lot. One part of this slowdown is the overhead to record the information internally, but another part is because I have to overload lots of opcodes. (Opcodes are PHP's internal execution units, similar to assembler instructions) They are always overloaded even if code coverage is not used, because it's only safe to overload them for the whole request.
These changes were from a combination of contributions from Taavi Burns and a new ini setting that will allow you to enable or disable the code coverage in XDebug. Benchmarking shows a good amount of time reduction in coverage runs - dropping anywhere from a few seconds to over a minute. He also mentions the idea of "modes", shortcuts to predefined settings for different types of reporting (like "profiling" or "tracing").