The main problem is that the internal buffer size for OpenSSH is limited to 2MB (actually it's closer to 1.25MB in real world terms). This is pretty good for a lot of home users. However, HPN-SSH was written for the HPC communuity where we have have BDPs of well more than that (8MB is not uncommon). So if you have a BDP of more than 2MB HPN-SSH is still going to be a win for you.