In this paper, we take a close look to understand the cloud-based file synchronization and collaboration systems. Using the popular Dropbox as a case study, our measurement reveals its cascaded computation and communication operations that are far more complicated than those in conventional file hosting. We show that this serial design is necessary for the cloud deployment, which effectively avoids the possible task interference inside the computation cloud; yet it also leads to higher service variance across users. Even worse, in a collaborative file editing session, users' updates would be discarded without any warning. The drop rate is unfortunately related to the slowest collaborator, which severely hinders the system scalability and user satisfaction. We further investigate the root causes of this phenomenon as well as other performance bottlenecks and offer hints for practical improvement.