Just in case it helps the troubleshooting, I've just checked an XP (32bit, Pro, SP3) installation of Scribus 1.4.3 on an OSX (10.6.8 ) BootCamp partition - no LAN connection but with internet via WiFi, Qt not installed - and the "Open" dialog works just fine. No delays.
However, I was wondering, and this is a long shot: Have you recently purchased a Western Digital external drive?
These sometimes have a habit of going to sleep at what seems like random times - even on desktop machines, and you can't change the settings without doing some really scary techie stuff. Once asleep they can take quite a while to wake up, even if you're not actually accessing them. So anything accessing any drive could show a delay while the drive spins back up to working speed.
(I wish I knew this before buying mine but it's been a few years so I'm sort of used to it by now, but it's still annoying when everything just stops for a while. It's nothing like 30 seconds but it breaks "the flow".)