Can you expand on your reasons for making that claim - perhaps with some benchmarks?
On Linux it uses an epoll native driver and is asynchronous. The framework makes it possible to write proxies in a few lines.
If you want to beat netty by a significant margin you'll probably need to use kernel bypass
Netty is also a lot easier to extend and more portable
If I had to pick something in the C/C++ space to implement a custom proxy, I would probably stick to something where I could find a similar list of established high volume real world users. Facebook's Proxygen, or some customized HAProxy maybe.