Performance drop when using timezone-aware DateTimeIndex
It seems that pandas.DataFrame operations on Index with timezone-aware dates is order of magnitude slower than on regular datetimes.

for a 500k datetimes created with pandas.date_range, and using DataFrame.shift() to compute deltas between dates, timings goes from **17ms** for standard datetimes to **16seconds** for timezone-aware datetimes.  

I don't understand why it is so slow with timezones objects.

I already posted a complete message related to this behavior on stackoverflow yesterday :
http://stackoverflow.com/questions/30385481/performance-of-timezone-aware-pandas-datetimeindex

I'm using latest pandas 0.16.1 from Anaconda, and latest numpy 1.9.2
