We are doing our best. So far, the performance and usage-of-resources are not the fields where we put the most of our effort. Additionally, there are two main issues to consider:
The first is about our security features: encryption and integrity check require some computation, hence our clients will have an higher load compared to Dropbox. Moreover, some of the magic performed by Dropbox to improve performance, cannot be combined with some of the security measures that we enforce.
The second is about the firepower of and age of the products you are comparing. Dropbox has a quite large and amazing team ( https://www.dropbox.com/about ) as well as few years of product development and testing on a large and impressively growing user base. The comparison with our team ( http://blog.filerock.com/founders/ and http://blog.filerock.com/dev-team/ ) and our time on this project so far, would be like throwing a cobble to the Ayers Rock and see who moves.
We are aware of the feature you mentioned: for instance, we already went for the filesystem monitoring using the API provided by the different platforms, but it turned out that they didn't behave as we expected on all platforms, in particular they made hard to get correct results needed by our integrity check technology. So we had to choose where to concentrate our effort and decided to put it on our distinctive features. So we switched to a simpler (and inefficient) approach that currently guarantees the correctness we need on each supported platform. As soon as possible, we plan to mix our current approach with the aforementioned usage of the different platform APIs to increase performance but still keeping our integrity check mechanism working properly.
Of course in the long term we aim to be as efficient as possible, and Dropbox is definitely the one to run after on this perspective. However, this will need time and resources. The good news is that having an open source client, anyone can contribute suggesting an improvement, or even implementing features that are not yet in our short-time roadmap (e.g., on-LAN files sync).