With both SMB (Windows File Sharing) and AFP (Apple File Sharing), low latency connections are key to achieving good performance. This is of course assuming that you already have a connection with reasonable bandwidth in between the two VPN endpoints (this can be easily verified by transferring a larger file through HTTP or FTP).
If you are using the Finder and the issue is mostly with listing folders (but not so much when actually copying files), try turning off icons/icon previews in the Finder's View Options (Cmd-J).
If you experience performance issues both when listing folders and when transferring files, your aim should be to reduce latency. Some ways to reduce latency include:
- Avoid high-latency Internet uplinks (e.g. satellite, some types of wireless providers, line aggregation, ...).
- When using a DSL line, see with your ISP if you can get "fast path" enabled (= interleaving turned off). ISPs sometimes market this as an option for online gaming, but it's also very helpful for AFP/SMB or connecting to some types of database backends.
- Make sure VPN traffic is appropriately prioritized in order not to be slowed down by someone else using the same Internet connection.
To measure latency between the two endpoints of the VPN, use ping to a host on the other end of the connection (or, when pinging from the client end, ping the VPN gateway). For your convenience, VPN Tracker has a ping utility built right in, it can be found in the Tools menu.
To measure latency of each endpoint's individual uplink, it's often helpful to do a ping the local router (to make sure there are no unnecessary latencies introduced in the local network) and the ISP's first router (to get an idea if enabling fast path or switching ISPs may be a suitable measure to decrease latency).
If you cannot reduce latency further:
If you are in a situation where you cannot reduce latency any further (or where the base latency from the distance between the two endpoints itself is so large local measures won't make much difference), consider switching to a file transfer protocol that is less vulnerable to latency, e.g. WebDAV or FTP, or use measures such as reducing the number of files and folders in a hierarchy to increase performance.