Add a FAQ about slowness due to PyTracer

1 files changed, 13 insertions, 0 deletions

diff --git a/doc/faq.rst b/doc/faq.rst
index 78db591..d7ae364 100644
--- a/doc/faq.rst
+++ b/doc/faq.rst
@@ -50,6 +50,19 @@ If you are using the :ref:`API <api>`, you need to call coverage.start() before
 importing the modules that define your functions.
 
 
+**Q: Coverage is much slower than I remember, what's going on?**
+
+Make sure you are using the C trace function.  Coverage.py provides two
+implementations of the trace function.  The C implementation runs much faster.
+To see what you are running, use ``coverage debug sys``.  The output contains
+details of the environment, including a line that says either ``tracer: CTracer``
+or ``tracer: PyTracer``.  If it says ``PyTracer`` then you are using the
+slow Python implementation.
+
+Try re-installing coverage.py to see what happened and if you get the CTracer
+as you should.
+
+
 **Q: Does coverage.py work on Python 3.x?**
 
 Yes, Python 3 is fully supported.