VirtualBox NAT vs Bridged Adapter

I’ve encountered networking problem for VirtualBox guest OS (Windows XP) several times. It’s very slow to access internet. It appears that the default NAT doesn’t play very well. The problem was solved by attaching the network adapter to Bridged Adapter instead. To do so:

  1. Shut down the guest OS
  2. Go to Settings - Network
  3. From the Attached to drop down list, choose Bridged Adapter
  4. Start the guest OS and try it
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I mainly use VirtualBox on a Mac. I have friends who run it on Arch Linux and they also have this issue.

Why Sun made the decision to attach a virtual machine a slower network adapter?