Antony,
Generic suggestions: (1) some kind of network or directory services issue (DNS, authentication, etc.) (2) some kind of I/O problem, e.g. dodgy hard drive, corrupt disk directory (maybe permissions problem -- though I can't quite see how). In particular, the "lookupd" background tool could stall and cause weird system-wide delays if there are problems with the aforementioned. Check settings in the Directory Services utility application, Network Preferences, etc. (And check log files for any error messages, crashed lookupd, etc, if you know where to look.)
