Optimize Countrows - GroupBy

Hi everyone!

I want to optimize a measure that I´ve calculated but I´m getting trouble trying to get the same result with different coding approaches.

In my data model I have something like this:

Fact_Table connected to Product dimension, to Time dim, Customer dim, Stores dim.

I want to calculate how many unique products I have in the combination Customer - Stores. 

Example:

Customer A

-Store A: 4 unique products.

-Store B: 6 unique products.

Customer B

-Store 01: 10 unique products.

-Store 02: 4 unique products.

When adding in a Table visual Customer Name + Measure I should see:

Customer A: 10 

Customer B: 14

I´ve managed to get this result doing this but performance sucks :(

COUNTROWS(
    GROUPBY (
        Fact_Table,
        'Customer'[Customer Name],
        'Store'[Store Name],
        'Product'[Item ID]
)
)