Generators are slow

f6448959 (yield statements) introduced a performance issue. I don't know to what extent this is due to the implementation or due to slowness of Python's generator implementation.

Timings of the test_multirate test run using the Python code generator (Python 3.2):

• before introduction of yield: 68.1 seconds
• after introduction of yield: 71.26 seconds

This is about 5% slower, which I'm not happy about.