When I checked the official MiUI forum, I found that there are a couple of complaints indicating the issue of slow download. If you are using a router for WiFi, check if the issue is with your router and phone. In some cases, certain sort of incompatibility between the router and the phone can cause such issues. Check it with any other router - may be at your school or office ( as the case may be).
Though I could find a few complaints, Xiaomi does not seem to be coming up with any update on the same. I would advice you to contact your authorised service center with the proof your tests.
One of the best options before opting for contacting service centre consider updating the firmware to the latest one. Xiaomi has been releasing frequent security updates on almost a monthly basis. Updating your phone to the latest available for your region may help you solve your problem.