https://github.com/EugenMayer/docker-sync is a very efficient workaround. It’s pretty new but a lot of works was done those past days and more is coming: https://github.com/EugenMayer/docker-sync/pull/64
I’ve just confirmed the performance problem on beta20.
I last tested on beta6 and I really struggle to find any significant difference on a large Rails project. Disappointed, to say the least.
Have a look at the new http://docker-sync.io release guys. No need for unison/unox anymore while providing native performance with far less CPU usage: https://github.com/EugenMayer/docker-sync/wiki/4.-Performance