Chris Webb crosses fact tables:
If you have a DirectQuery fact table in Power BI you can use user-defined aggregations to improve query performance; querying a smaller, summarised copy of your data in an Import mode aggregation table is always going to be faster than querying a large fact table containing all your detail data that is in DirectQuery mode. What’s more a composite model like this can have a much smaller footprint in memory than a model where all your tables are in Import or Direct Lake mode, which means you can use a smaller Fabric capacity SKU. However, in some cases you can take the same tables that you would use to create a composite model like this and solve the same problem slightly differently without using aggregations.
Click through for an example of this.