Scenes that use classic Maya dynamics (for example, particles, cloth, rigid bodies) can cause the Evaluation Manager to switch out of Parallel mode to Serial. You can discover which dynamics nodes in a scene disable the Evaluation Manager with a mel script.
Enter the following script into the Command line to return a list of the nodes that are deactivating the Evaluation Manager:
evaluator -name "dynamics" -valueName "disablingNodes" -q;
Once you have identified the nodes that are affecting performance, disconnect or temporarily delete them.