You want to try and keep your joining field values as short as possible, for example
'Value1' & 'Value2' & 'Value3' as myKey
The length of the field is quite high 18.
Autonumber('Value1' & 'Value2' & 'Value3') as myKey
This is taken from the help:
Returns a unique integer value for each distinct evaluated value of expression encountered during the script
execution. This function can be used e.g. for creating a compact memory representation of a complex key.
In order to create multiple counter instances if the autonumber function is used on different keys within the
script, an optional parameter AutoID can be used for naming each counter.
(If you'd like further assistance you might want to upload your QVW or at least a screen grab of the data model.
Rereading your first post I'm now unclear what you are asking for, your question is vague.
Perharps a metaphor would be if you took your car to a garage and asked the engineer "Can you make my car work better?"
"I thinks that this way of creating my key break down the performence of my report.
can you advice me how to develop properly my report ?"
Are you suggesting you have not devloped your report* properly?
What do you mean by "report" do you mean the data model or the UI?
Are you experiencing performance issues, what are they, what are you expecations?