This is caused because, by default in the Xiaomi rom (China or global) is also the search for frequencies also Chinese, this involves an intensive use of the network module that continues in a way to find these Chinese frequencies, which of course in our country , just as all EU countries do not exist, leading to greater use of autonomy.
Xiaomi is already aware of the problem and will certainly be resolved, when it is not known.
In the meantime, you can solve the problem by yourself, as follows:
Enter the following USSD code *#*#4636#*#* in the dialer.
Set the LTE / WCDMA mode to both SIMs
In this way, only the European frequencies will be used, eliminating the battery drain.