I have bought a new mi 5 mobile My older phone internet was too fast. When I transferred my sim into new phone, the internet is too damn slow. It's slower than tortoise. What should I do?
The only thing I can suggest is to check and make sure that your phone is set to 4G (preferred) mode in Network settings.

Another thing worth considering is to check with your operator that the correct access point is being used.

And can you tell which was your previous phone? Depending on the device and its OS, it is very possible that it consumed lesser data whereas your Android's browser uses more data. Consider using Opera Mini or other browsers which compress pages before delivering to your device.