I noticed a severe performance degradation in the execution of the pipeline sh step when the agent is connected via the ssh-slaves plugin. 

I performed extra testing on a standalone jenkins installed from docker image jenkins/jenkins:lts with a number of different versions of the ssh-slaves plugin.

Attached is the Jenkinsfile I used to test this.

With 10 iterations of the sh step, versions 1.26 takes 14-15 seconds while version 1.27 takes 24-27 seconds.

That is without taking into account the first build after the agent has started which always takes longer (22 seconds in 1.26 and 40 seconds in 1.27)

I have also tried "Launch agent via execution of a command on the master" which performs in a way comparable to the ssh-slaves plugin in version before 1.27.

The only workaround I found is to keep the ssh-slaves plugin at version 1.26.