Netty is significantly slower than OkHttp

### What version of gRPC-Java are you using?

Tried this with all of:
- 1.24.1
- 1.26.0
- 1.27.0

### What is your environment?

Both:
- running locally on MacOSX 10.14.4
- running in Kubernetes - Ubuntu 18 image on Ubuntu 18 workers

JDK: Zulu, java 8

### What did you expect to see?

Netty & Okhttp clients should have relatively similar performance.

### What did you see instead?

OkHttp is about 5x faster than Netty.

### Steps to reproduce the bug

See methodology here: https://github.com/grpc/grpc-java/issues/6685

In those experiments, a server-side streaming RPC is set up where Netty is benchmarked at receiving 40k messages/sec while OkHttp receives 200k messages/sec (with low latency).