Virtualbox: SLOW shared folders

I have been using Virtualbox on Linux machines, with XP as a guest, for a while now. The only reason that I haven't moved away from Windows altogether is that I still use Paintshop Pro as a photo editor (Gimp is useless to me because of the way it handles TIFFs). One of the things I liked about Virtualbox was the ability to use shared folders to share files between the host and the guest. However, I have become increasingly frustrated by the speed at which large graphic files get loaded using these shared folders. This has gotten worse with the newer versions of Paintshop Pro, so I decided to do some speed tests with the different versions, and with different ways of accessing the same file. The file is a 22.9MB tiff.

	v9	x2	x3
Direct	5s	13s	11s
SMB	12s	18s	31s
SFolders	37s	3m 6s	4m 27s

Direct is the guest machine's C:-drive
SMB is using SAMBA on Linux to share the folder with XP
SFolders is using Virtualbox's shared folders

From this example, the file handling of the software has clearly changed with the newer versions, but the time taken to open files like this when using Virtualbox's shared folders is unacceptable.

Another example is using Canon's Digital Photo Professional to access a directory containing 41 RAW files from a Canon 40d:
Direct: 5s
SMB: 13s
SFolders: 49s
The time given here is the time taken from when the directory is opened until all thumbnails have been built.

Update
I've tried similar tests with an external USB hard drive attached directly to the VirtualBox (through Devices>USB devices). Here using VirtualBox v3.2.8 under Kubuntu 10.04 with Windows XP as guest, with the same 22.9MB tiff used in the earlier tests. Again, the time given is the time taken for the file to load completely in Paintshop Pro v9:
Direct: 5.0s
USB drive: 6.3s
SMB: 13.5s
SFolders: 29.7s