Not sure if you are using my net balancing and QoS patch or not but… I would expect your speed test to match the data rate of one of your interfaces, not the sum of all of them.
Internet traffic is between specific IP addresses. And TCP connections over IP require fixed end points. The end result is that once your router, Zeroshell in this case, picks an interface to use for the speed test it should send all the traffic for that connection by that interface. So net balancing only balances connections between interfaces. It does not and cannot balance data between interfaces except in the roughest possible way. Net balancing will help you if you have multiple TCP connections going to many different destinations, but not multiple TCP connections going to the same destination.
Further, if you are using my patch or a non-Zeroshell router, connections can be “sticky”. That is if a connection to server A used interface 1, the subsequent connections to server A will use that same interface. You need this so that sites that use HTTPS will work.
If you wish to aggregate multiple interfaces in a way that will balance the data rates on multiple links then you will need to look into “bonding” rather than “net balancing”. This allows one IP address to be shared among multiple Ethernet adaptors. Bonding only works on interfaces that look like Ethernet adaptors to the Linux operating system. And bonding takes set up on both ends so you will need a service provider or other entity on the other side to set up a compatible configuration for you.
You can make your two different ISP connections look like Ethernet interfaces by setting up VPNs on each interface to an appropriately configured service provider (the VPN software simulates an Ethernet interface to Linux) and then bond the multiple VPN interfaces into one logical Ethernet interface to be used for actual traffic.
So to make bonding work you will need a server some place on the Internet that you can route all your traffic through. And that server will need to be set up to allow you multiple VPN connections and then bond those VPN connections into one logical Ethernet interface. Needless to say most people don’t have a way to do this. I’ve only really seen this done for setting up remote office access to headquarters. This works because you have one IT department that is responsible for setting up both ends of each link and who can dedicate appropriate equipment to the service.