Why does latency increase significantly on Node JS server when using Node's cluster-service?

Question

I measured latency on 1000 requests to on of my REST endpoints (Node JS server on Express running locally) and 99% of requests were served under 180.9 ms.

If I run the same test with multiple cores using the cluster-service npm package, 99% of requests were then served in 6498.2 ms. I don't understand why.