Hello,

If you select the "Data stored in an external table", you just have to specify into which database table and columns.
The options list of the cascade field will be filled with the content of the table.

So, for the dropdown field (the first one), you need to specify:
Database table name - The name of the table in your database
Column name for label - The column that contains the labels
Column name for value - The column that contains the values

And for the cascade field (the second one), you need to specify the three same fields + this one:
Column name for parent value - The column that contains the parent value (idprovincia in your case)

If it doesn't work, first try the two fields by doing a simple dropdown for each of them.
If that doesn't works, then maybe the name of the database or the name of the columns are not correct.
If that works, change the type of the second field to cascade and try it normally.

I hope this will help you.

Best regards.

If you found it too slow
You can create an SQL index on the parent value (for the cascade field)

Except that there is nothing to optimize

What do you call slow more than 1 sec to get the children values ?