$group is taking 2x as long as collection.group()

This came from a DISQUS comment on the "Aggregation Framework" page:
I'm testing MongoDB 2.1.0 in order to evaluate the performance of the new aggregation framework. I'm wondering why it's 2x slower in my use case.

Here is the code I used before version 2.1.0 (using Python and pymongo):

db.customers.group(

{'segment': True}

, None,

{'count': 0}

, "function (obj, prev)

{ prev.count ++; }

" )

Here is the same computation using the new aggregation framework:

db.command('aggregate', 'customers', pipeline=[ {'$group' : { '_id': '$segment', 'count':

{'$sum': 1}

}} ])

On my computer with my dataset, the first version runs in ~1 s, the second version in ~2.5 s. Is it expected or am I doing something wrong?