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.
Within the SpiderOak application, this problem is actually caused by a bug in QT, the open source graphical toolkit from Nokia that SpiderOak uses. The bug is described here in QT's bug database: https://bugreports.qt.nokia.com/browse/QTBUG-13182
This bug has already been addressed in the newest version of Qt 4.8, which is now in beta and is slotted to be included with SpiderOak as soon as it is released generally.