After I've upgraded my Hudson installation to Warnings plugin 2.3 the build time
of my project increased from ca. 3 minutes to > 7 minutes.

The console shows that the Maven build itself doesn't take longer:

"[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3 minutes 4 seconds
[INFO] Finished at: Tue Feb 24 11:44:26 CET 2009
[INFO] Final Memory: 140M/890M
[INFO] ------------------------------------------------------------------------
channel stopped
[locks-and-latches] Releasing all the locks
[locks-and-latches] All the locks released
[WARNINGS] Parsing warnings in log file...
[WARNINGS] Found 68 annotations (0 new, 0 high, 68 normal, 0 low)
[WARNINGS] Don't changing build status, no threshold has been exceeded
No emails were triggered.
finished: SUCCESS
"

I've now disabled the warnings scanner and the build time went back to normal.