Jorge, you are likely going to have to attach the QVW on this one for someone to be able to try to give you some guidance. In these cases, folks generally need to see the data model as well as the context of the expression etc...
What I can tell you is the message you are getting indicates the calculation is taking up a lot of resources, which would lead me to believe that it is creating some sort of Cartesian product etc., and again, this is where folks will need to see your data model etc. in order to try to figure out what is going on.
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