merge_items speedup

redbo@precise-saio:~/swift$ git checkout master
Switched to branch 'master'
redbo@precise-saio:~/swift$ python merge_items_test.py
34.8510200977
redbo@precise-saio:~/swift$ git checkout speedup
Switched to branch 'speedup'
redbo@precise-saio:~/swift$ python merge_items_test.py
17.1159229279

Change-Id: I000dae4e8dd0f6bf546ebdae1e8ae69c06f5f44b