Shared folders are slow because they are a form of virtual networking and data has to be transmitted between the virtual and host machines just as it would over a real network.
I suspect the speed increase in that suggestion above is because the shared folders are no longer being used, but a local copy has been created on the vm!
Have you tried sharing a real host partition with the guest (see chapter 9.10 of the VBox manual)? If you keep the folders you need to share on this real partition you should have immediate access to them in both machines.