Dropbox is smart about using bandwidth. It will only use as much as it can without interfering with your normal Internet usage.
Specifically, Dropbox automatically throttles itself to 75% of your maximum upload speed to prevent any noticeable slowdown in browsing. Downloads are performed at the fastest download speed available.
These settings are customizable. If you'd like to change Dropbox's bandwidth settings, follow these steps:
Note: Setting your Upload rate to Don't limit or a higher number than your connection is capable of will probably cause all your other Internet activity to slow down significantly.
Dropbox for Business users: If you are signed in to both your personal and work Dropboxes on your computer, the network preferences will apply across both.