Yes, it was primarily because of the performance. According to these benchmarks, unicorn totally choked: "... At the bottom we have Twisted and Gunicorn ..."
Look at the comment, the twisted code is wrong. By default the twisted reactor is multiplatform, but you can imporve performance for your specific platform. It's in the doc.
If you run Twisted under linux and you should, use the epoll reactor.
http://nichol.as/benchmark-of-python-web-servers