I am attaching a simpler and hopefully more revealing test, speed_test.py.  This test repeatedly draws a full circle at various speeds and prints the time spent.  Here is the result:

# python3.1 speed_test.py
         0: 0.03
         1: 6.61
         2: 4.32
         3: 3.78
         4: 3.23
         5: 3.21
         6: 2.65
         7: 2.72
         8: 2.66
         9: 2.86
        10: 2.67

Note that time decreases as speed changes from 1 to 4 as expected, but then changes unpredictably as speed changes from 5 to 10.