The early betas focussed on feature completeness rather than performance for filesystem sharing. In particular, we have implemented a new "osxfs" that implements bidirectional translation between Linux and OSX filesystems, including inotify/FSEvents and uid/guid mapping between the host and the container. Getting the semantics right took a while, and all the recent betas have been steadily gaining in performance as we implement more optimisations in the data paths.
If you do spot any pathological "spinning cases" where a particular container operation appears to spiking the CPU more than it should be in, we'd like to know about it so we can fix it. Reproducible Dockerfiles on the Hub are particularly appreciated so that we can add them to the regression tests.