It may be that you do not use the Xiaomi standard charger and data cable, if you use other models of charger or data cable will charge slowly, or the charger head and data cable do not match, it is recommended to replace the data cable, it is best to use the original data cable and charger head.
There is also your charging environment will also affect the charging speed of the phone, the temperature is too high or too low will cause.
Small original charger, if the charging is slow, there are basically no problems, one is that the wiring amount inside has reached, then what about this, resulting in how much current quality can not be fast charging?
This is one of the most likely things to happen, including the line above have to carry out a own check, the second point to check the line