In my quick tests, default dict access is a tiny bit slower, but not enough to matter.

```python

In [69]: import collections

In [70]: d = {}

In [71]: dd = collections.defaultdict(int)

...

In [74]: d['a'] = 1

In [75]: dd['a'] = 1

In [76]: %timeit d['a']
10000000 loops, best of 3: 47.3 ns per loop

In [77]: %timeit dd['a']
10000000 loops, best of 3: 50.5 ns per loop

In [78]: %timeit d['a'] += 1
10000000 loops, best of 3: 102 ns per loop

In [79]: %timeit dd['a'] += 1
10000000 loops, best of 3: 104 ns per loop

```