Speed up the detection of these features

[laowantong]

paroxython collect --log path/to/django-master/

... produces the following performance report:

Labelling 2668 programs.
                                                                                                                    
Elapsed times by features (in seconds)
--------------------------------------
333.6141         TOTAL
 65.2732         node
 26.0090         if_then_branch
 16.3087         index
 10.1161         corrective_conditional_assignment
  8.1330         function_without_arguments
  7.9829         nested_function
  7.6321         free_call_without_result
  7.1866         decorated_function
  6.6105         method_chaining
  6.2202         concatenation_operator|replication_operator
  6.1109         function_decorator
  6.0286         string_formatting_operator
  5.8143         free_call_without_arguments
  5.6189         function_argument_flavor
  5.3779         nested_index
  5.0084         composition
... below 5 seconds ...

See if anything can be done about the slowest features.