I confirm the same. 1.538 here.
I had to replace the trilead-ssh2-build217-jenkins-3.jar library with older trilead-ssh2-build214-jenkins-3.jar from 1.535 which significantly improved performance.

I changed from 217 to 214 and the download speed is changed from 100KB/sec to 210KB/sec.

To clarify, you're downloading

• archived artifacts from Jenkins master, or
• files in the workspace from master, or
• files in the workspace from a slave?

(please try all of the above that don't apply and report speed for those as well for comparison.)

How is the slave connected to the master?

Do you use the "zip" functionality, or download single files?

I'm download from the ear/war/jar built by Maven, I think it's the archived artifacts from Jenkins master directly.

Any way you could set up a test instance, verify that it works well in that test instance using 1.529, doesn't work well in 1.537, and then try to narrow it down to the one release that caused the issue?

FWIW there's no SSH involved when you're downloading archived artifacts from Jenkins master.

In my environment, when I downgrade trilead-ssh2-build217-jenkins-3.jar to trilead-ssh2-build214-jenkins-3.jar, it's faster than before.

It would be good if you could put enough logging on the SSH server side to see if there is a different cipher being used with the newer lib

A relevant conversation in the dev list

FYI. I had the same issue on my Jenkins instance, both 1.609 and 1.638 (running on Ubuntu 12) with HTTPS/SSL in Jenkins.
Once I've reconfigured Jenkins to run behind Apache, moved SSL to Apache and removed it from Jenkins configuration, download speed increased by 20+ times on huge files.
Hope this helps.