The slowness can happen on Windows when folder redirection is used to specify a different location (usually on a network drive or similar) for folders such as My Documents. This can also happen on Windows if there is a floppy drive attached - or even if there isn't a physical floppy drive, but simply an A: drive option in your Device Manager.
To correct this slowness, open Windows Device Manager and see if you have a listing for a floppy A: drive. If you do, delete or disable this drive, then try reopening SpiderOak. Switching between tabs should now be noticeably faster.