Add section about performance

doc/performance.rst

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+Performance
+===========
+
+For performance benchmarks, it is desirable to achieve as high performance as feasible with PythonPDEVS.
+By default, PythonPDEVS offers a lot of features, most of which have a (very) negative impact on actual performance.
+While this might not be too noticable on small or medium-sized models, huge models will become unbearably slow in the default configuration.
+
+There are two proposed tweaks, which on average increase performance by a factor 12.
+
+Minimal Simulation Kernel
+-------------------------
+
+Using the minimal simulation kernel disables most performance-affecting features, thus significantly increasing performance.
+Note, however, that most of the advertised features will simply not work in this configuration.
+This option is preferred for a fair comparison with other tools, as the PythonPDEVS defaults were chosen for usability, rather than performance.
+
+To use the minimal simulation kernel, simply replace all imports of PythonPDEVS to include the file ``pypdevs.minimal``.
+This includes the ``AtomicDEVS``, ``CoupledDEVS``, and ``Simulator`` classes, and can thus be used instead of the ``pypdevs.DEVS`` and ``pypdevs.simulator`` files.
+Both simulation kernels have the same interface, so switching can be done merely by changing the import statements.
+When all features are desired, for example for debugging, the imports can easily be changed again to the full version of the PythonPDEVS simulation kernel.
+
+Use of PyPy
+-----------
+
+Much of the performance related problems in PythonPDEVS are related to the Python interpreter itself.
+We therefore recommend the use of PyPy as an alternative drop-in replacement for the default CPython implementation.
+PyPy is the main development platform of PythonPDEVS as well.
+
+Note that at the time of writing, PyPy does not always behave well on Windows systems.
+It works perfectly fine on Linux.