Besides a debugging and development aid, Xdebug also implements the back-end code to provide code coverage information for use with PHPUnit. 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.

The upcoming Xdebug 2.2 has a few changes to improve code coverage performance. First of all, the speed with which information about code coverage is recorded has been improved by contributions by Taavi Burns. And secondly, Xdebug 2.2 features a new setting, xdebug.coverage_enable with which it is possible to disable the overloading of code coverage specific opcodes. By default that setting will still be on.

Just to show how those improvements have effect on the speed, I've run a benchmark (running Apache Zeta Components Graph's tests) to compare Xdebug 2.1.2 against Xdebug 2.2-dev.

The results are as follows:

Version	With CC	Without CC	With coverage_enable=0	Without Xdebug
2.1.2	06:44	00:49	not available	00:32
2.2-dev	05:37	00:48	00:46	00:32

Another benchmark, run by Ross McFarlane gives: @derickr Based on 3 runs each, execution of 254 tests with 1070 assertions has dropped from 19s to 12s. Nice work!.

In Xdebug 2.2 I would also like to introduce modes. Each mode will set a default configuration for Xdebug's setting to be the most optimal for that specific tasks. Examples of tasks could be: "profiling", "debugging" or "tracing". Let me know (in the comments) whether you think that's a good idea.