The SDN control plane provides the data plane with packet forwarding rules and thus defines its behavior. Understanding the performance of the controller is critical to assess the overall performance of the network. Although OpenFlow-based controllers have been well investigated in this regard, controller performance studies based on P4Runtime, which is the de-facto implementation for the southbound control of programmable data planes in P4, are still missing.In this work, we implement a benchmarking tool for P4Runtime-based controllers, apply the tool to evaluate the performance of the ONOS controller running in both OpenFlow and P4Runtime mode, and identify processing bottlenecks in the P4Runtime implementation. In addition, We propose a code patch for the implementation, which shows a 17 % improvement in the achieved packet rate.