First, consider the problem of charging cable or charger, you can find another quality no problem charging cable and charger to test and exclude;
Two: It may also be the problem of mobile phone charging port, if the problem is still tested with a new charger and line, it should be considered that the problem of mobile phone charging port (charging small board)!
Three: Finally, the problem of the mobile phone motherboard, this possibility is relatively small!
The above is the answer of how Xiaomi mobile phone fast charging becomes slow charging.
It may be because the charging head you use is not original, in the case of using a non-original charging head is such a situation, if the use of the original or slow charge that may be the aging of the charging line.
You should not use the original charger to turn into a slow charge. Using the original charger will turn into a fast charge.