Testing VPN throughput using a remote file share is usually not a good idea for two reasons:
The first reason is the file sharing protocol itself. File sharing protocols like SMB, AFP, or NFS have been designed for local networks that are fast, reliable, and have a very low latency. The Internet on the other hand is slow (at least the connection to it), unreliable and has a very high latency. For realistic results, you need to use a protocol that was optimized for such a situation, like HTTP or FTP.
The second reason is the implementation of the file sharing protocol. Today most file shares use SMB, the Windows file sharing protocol. Apple has its own implementation of that protocol but this implementation is anything but good. While the SMB 3.x implementation is already poor, the SMB 1.x/2.x implementation (compatibility mode) is horrible, and for several reasons macOS will often fall back into that compatibility mode. When testing with a local NAS file share, we got 28 MBps using SMB 3 and only 18 MBps using SMB 1, compared to 50 MBps using AFP.
If you have a Mac at the remote side, it’s pretty easy to setup a benchmark HTTP server. All you need is to open the standard application “Terminal” (use spotlight to find it) and then run the following set of commands (every command is confirmed by Return/Enter):
dd count=1048576 bs=1024 if=/dev/random of=1GiB.dat
php -S 0.0.0.0:8080
The First command creates a new directory, the second one enters that directory, the third one creates a 1 GiB data file filled with random data, the last one starts a primitive HTTP server that serves the content of the current directory at port 8080. Now your VPN users can just open this address in Safari (or any other browser):
a.b.c.d” is the IP address of the Mac where you just typed the commands above. By watching the transfer speed in browser, you get a good idea of how capable your VPN is. Of course, this is limited by many factors, like the speed of your local Internet connection, the speed of the remote Internet connection, and the CPU power of the VPN gateway (which is usually far less than the CPU power of a Mac).
To clean up after the test, activate the terminal window again and hit CTRL+C to stop the HTTP server, and finally run the following two commands:
rm -r /tmp/www-bench