Our Jenkins instance runs 1.553 (since two weeks, was an three year old version before) on Linux with 10 slaves: 6 Linux connected through SSH and 4 windows with WebStart agent started as services.
When no job is running, CPU is almost 0. Correct.
When only two or three jobs are running, Jenkins CPU raises between 70% to 160%. atop declares 96% of CPU time spent in IRQ (even with "no" disk access), most CPU consumption is considered as system time. On average since boot, the master node consumes 100% of one CPU. Even if it has 4 CPUs, job execution time is between 2x and 3x compared to older version.
I configured JMX and did a quick CPU profiling. Top consumer threads are unnamed and are all related to SSH communication
So there is chance the "SSH agent plugin" is concerned.
I am ready to do deeper analysis on my system if required, and of course to test patches.