I'm using the SpeedMod Mutator and have the gamespeed set at 25%, the 20 minute matches are logged as only taking 5 minutes (via the chatlog, and connection log)
I have the latest UTStatsDB version from SVN (build 118)
Are you checking the gamespeed by looking for the specific speedup/slowdown mutators that come along with UT3?
I can see that the match-length are reported as 20 minutes in a specific match overview, (Match Limit: 20 minutes)