I have a very similar problem.
I develope a application with Jsch v44 that put's via sftp a big files >Gb
in a AIX platform and the conection must be with Proxy.
With system SFTP command I obtain around 400kb of throughtput, but with Jsch
Finally I do the same test with a trial JScape J2SSH API, a comercial Java
implementation of SSH2
and I obtain the same values that SFTP comand, 400kb!
I suspected that are something is wrong in the transmision algotithm, but I
don't have any idea.
When I try more api's and post the results.