Sorry, but this issue is on Dropbox side.
Dropbox's support team says that the connection speed will vary from place to place. Your connection speed to Dropbox depends on the routing you get between your ISP and Dropbox's servers, and may be slower than your ISP's rated speeds.
Sometimes resetting or retrying your connection gets you a different route and better speeds, but that is outside of our and Dropbox's control. Some ISPs also throttle sustained connections so if you see an initial high connection speed followed by lower speeds, that could be the reason.
Also, we can recommend you to contact Dropbox's support team to check the issue related to your routing.