When the number of parallel branches in pipeline increases a lot, the performance of the pipeline decreases significantly. There appears to be an n^2 algorithm somewhere.

~200 branches - okay
4k branches - glacial

Attached is a stack trace taken while the behavior was exibited. I'm pretty sure this ends up n^2:

"Running CpsFlowExecutionOwner[corex_full/54:corex_full #54]" Id=421 Group=main RUNNABLE
at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.getCurrentHeads(CpsFlowExecution.java:703)

• locked org.jenkinsci.plugins.workflow.cps.CpsFlowExecution@6ebdc0a3
  at org.jenkinsci.plugins.workflow.support.actions.LogActionImpl.isRunning(LogActionImpl.java:152)
  at org.jenkinsci.plugins.workflow.support.actions.LogActionImpl.access$000(LogActionImpl.java:66)
  at org.jenkinsci.plugins.workflow.support.actions.LogActionImpl$1.onNewHead(LogActionImpl.java:93)
  at org.jenkinsci.plugins.workflow.cps.CpsFlowExecution.notifyListeners(CpsFlowExecution.java:1081)
  at org.jenkinsci.plugins.workflow.cps.CpsThreadGroup.notifyNewHead(CpsThreadGroup.java:381)
  at org.jenkinsci.plugins.workflow.cps.FlowHead.setNewHead(FlowHead.java:119)
  at org.jenkinsci.plugins.workflow.cps.DSL.invokeStep(DSL.java:171)
  at org.jenkinsci.plugins.workflow.cps.DSL.invokeMethod(DSL.java:126)
  at org.jenkinsci.plugins.workflow.cps.CpsScript.invokeMethod(CpsScript.java:108)

The code walks the heads every time a new head is added.