I use the EC2 plugin (with no delay provisioning) to spin up nodes for my pipeline jobs.  I've noticed in either a recent plugin or Jenkins update (last 3 months), that when a new node is needed, there often seems to be a delay after the node is launched before the job will notice it can continue.  

For example:

• Master idle with no nodes running
• Git commit triggers build
• Job triggers creation of an EC2 instance (30 seconds).
• Node init runs (2 minutes).
• Job should now continue - agent now shows a build queue, but can sit idle for several minutes.  Checking the log output of the job shows an old message of "Waiting for next available executor" with a timestamp relating to before the node was requested.

If another job needing the same label is triggered in the mean time, this appears to remind the other job that it was waiting for a node.

Not terrible, but can double some build times, other jobs spin up more than one node during the build process.  Once the node is running, jobs seem to trigger as expected.