Based on my experience with using Orchestrator and something that very much depend on your usage its a good practice to either throttle your runbook activity up and down to ensure best performance.
To do this need to look at Runbook Server Throttling which is set to run 50 Runbooks parallel by default maximum.
Depending on your specification of your server this figure might be too high or too low (will get a good indication anyways via taskmgr/perfmon) , anyways to increase or decrease the number of parallel Runbooks allowed to be run in parallel see below.
So the Runbook Server Throttling tool is the tool you need. To run go to
C:\Program Files (x86)\Microsoft System Center 2012 R2\Orchestrator\Management Server
and run the following command:
aspt <RunbookServer> <MaxRunningRunbooks>