I use every day rsync over SSH, and even between 7 to 10 years old computers it reaches the maximum link speed over 2.5 Gb/s Ethernet.

So in order to need something faster than rsync and be able to test it, one must use at least 10 Gb/s Ethernet, where I do not know how good must be your CPU to reach link speed.

For 7x faster, one would need to use at least 25 Gb/s Ethernet, and this in the worst case for rsync, when it were not faster on higher speed Ethernet than what I see on cheap 2.5 Gb/s Ethernet.

If on a higher-speed Ethernet the link speed would not be reached due to an ancient CPU that has insufficient speed for AES-GCM or for AES-UMAC, then using multiple connections would not improve the speed. If the speed is not limited by encryption, then changing TCP parameters, like window sizes, would probably have the same effect as using multiple connections, even when using just rsync over ssh.

If the transfers are done over the Internet, then the speed is throttled by some ISP and it is not determined by your computers. There are some cases when a small number of connections, e.g. 2 or 3 may have a higher aggregate throughput than 1, but in most cases that I have seen the ISPs limit the aggregated throughput for the traffic that goes to 1 IP address, so if you open more connections you get the same throughput as with fewer connections.