DateTime performance

Question:

When using DateTime for a large number of
instances, it becomes a serious performance
drag. 

A typical application for me involves things like
log files: I use DateTime to translate the timestamps 
in these files into a canonical format, and then get 
information such as "day-of-week" or "time-of-day" 
from DateTime. 

However, when working through a files with a few 
tens of millions of records, DateTime turns into a 
REAL drag on performance.

Is this expected behavior? And are there access
patterns that I can use to mitigate this effect? 
(I tried to supply a time_zone explicitly, but that
does not seem to improve things significantly.)

Best,

		Ph.